bug #35162 [Mailer] Make sure you can pass custom headers to Mailgun (Nyholm)

This PR was squashed before being merged into the 4.4 branch.

Discussion
----------

[Mailer] Make sure you can pass custom headers to Mailgun

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | fix #35095
| License       | MIT
| Doc PR        |

I can only assume that there was a mistake to use `$header->toString()` since it makes no sense to include the header name here. That is why this is a bugfix and not a new feature.

Without this fix my test will fail:
```
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'{"foo":"bar"}'
+'X-Mailgun-Variables: {"foo":"bar"}'
```

There are a few similar places for other mailer bridges that looks like they have similar issues. I dont know those providers very well. I can spend time researching them and confirm if there are bugs there too. But first I want to make sure a fix like this is accepted.

Commits
-------

43c1e77a88 [Mailer] Make sure you can pass custom headers to Mailgun

src/Symfony/Component/Mailer/Bridge/Mailchimp/Tests/Transport/MandrillApiTransportTest.php

@@ -13,6 +13,9 @@ namespace Symfony\Component\Mailer\Bridge\Mailchimp\Tests\Transport;
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Mailer\Bridge\Mailchimp\Transport\MandrillApiTransport;
+use Symfony\Component\Mailer\Envelope;
+use Symfony\Component\Mime\Address;
+use Symfony\Component\Mime\Email;
 
 class MandrillApiTransportTest extends TestCase
 {
@@ -41,4 +44,21 @@ class MandrillApiTransportTest extends TestCase
             ],
         ];
     }
+
+    public function testCustomHeader()
+    {
+        $email = new Email();
+        $email->getHeaders()->addTextHeader('foo', 'bar');
+        $envelope = new Envelope(new Address('alice@system.com'), [new Address('bob@system.com')]);
+
+        $transport = new MandrillApiTransport('ACCESS_KEY');
+        $method = new \ReflectionMethod(MandrillApiTransport::class, 'getPayload');
+        $method->setAccessible(true);
+        $payload = $method->invoke($transport, $email, $envelope);
+
+        $this->assertArrayHasKey('message', $payload);
+        $this->assertArrayHasKey('headers', $payload['message']);
+        $this->assertCount(1, $payload['message']['headers']);
+        $this->assertEquals('foo: bar', $payload['message']['headers'][0]);
+    }
 }

src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillApiTransport.php

@@ -111,7 +111,7 @@ class MandrillApiTransport extends AbstractApiTransport
                 continue;
             }
 
-            $payload['message']['headers'][] = $name.': '.$header->toString();
+            $payload['message']['headers'][] = $name.': '.$header->getBodyAsString();
         }
 
         return $payload;

src/Symfony/Component/Mailer/Bridge/Mailgun/Tests/Transport/MailgunApiTransportTest.php

@@ -13,6 +13,9 @@ namespace Symfony\Component\Mailer\Bridge\Mailgun\Tests\Transport;
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Mailer\Bridge\Mailgun\Transport\MailgunApiTransport;
+use Symfony\Component\Mailer\Envelope;
+use Symfony\Component\Mime\Address;
+use Symfony\Component\Mime\Email;
 
 class MailgunApiTransportTest extends TestCase
 {
@@ -45,4 +48,20 @@ class MailgunApiTransportTest extends TestCase
             ],
         ];
     }
+
+    public function testCustomHeader()
+    {
+        $json = json_encode(['foo' => 'bar']);
+        $email = new Email();
+        $email->getHeaders()->addTextHeader('X-Mailgun-Variables', $json);
+        $envelope = new Envelope(new Address('alice@system.com'), [new Address('bob@system.com')]);
+
+        $transport = new MailgunApiTransport('ACCESS_KEY', 'DOMAIN');
+        $method = new \ReflectionMethod(MailgunApiTransport::class, 'getPayload');
+        $method->setAccessible(true);
+        $payload = $method->invoke($transport, $email, $envelope);
+
+        $this->assertArrayHasKey('h:x-mailgun-variables', $payload);
+        $this->assertEquals($json, $payload['h:x-mailgun-variables']);
+    }
 }

src/Symfony/Component/Mailer/Bridge/Mailgun/Transport/MailgunApiTransport.php

@@ -114,7 +114,7 @@ class MailgunApiTransport extends AbstractApiTransport
                 continue;
             }
 
-            $payload['h:'.$name] = $header->toString();
+            $payload['h:'.$name] = $header->getBodyAsString();
         }
 
         return $payload;

src/Symfony/Component/Mailer/Bridge/Postmark/Tests/Transport/PostmarkApiTransportTest.php

@@ -13,6 +13,9 @@ namespace Symfony\Component\Mailer\Bridge\Postmark\Tests\Transport;
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Mailer\Bridge\Postmark\Transport\PostmarkApiTransport;
+use Symfony\Component\Mailer\Envelope;
+use Symfony\Component\Mime\Address;
+use Symfony\Component\Mime\Email;
 
 class PostmarkApiTransportTest extends TestCase
 {
@@ -41,4 +44,21 @@ class PostmarkApiTransportTest extends TestCase
             ],
         ];
     }
+
+    public function testCustomHeader()
+    {
+        $email = new Email();
+        $email->getHeaders()->addTextHeader('foo', 'bar');
+        $envelope = new Envelope(new Address('alice@system.com'), [new Address('bob@system.com')]);
+
+        $transport = new PostmarkApiTransport('ACCESS_KEY');
+        $method = new \ReflectionMethod(PostmarkApiTransport::class, 'getPayload');
+        $method->setAccessible(true);
+        $payload = $method->invoke($transport, $email, $envelope);
+
+        $this->assertArrayHasKey('Headers', $payload);
+        $this->assertCount(1, $payload['Headers']);
+
+        $this->assertEquals(['Name' => 'foo', 'Value' => 'bar'], $payload['Headers'][0]);
+    }
 }

src/Symfony/Component/Mailer/Bridge/Postmark/Transport/PostmarkApiTransport.php

@@ -84,7 +84,7 @@ class PostmarkApiTransport extends AbstractApiTransport
 
             $payload['Headers'][] = [
                 'Name' => $name,
-                'Value' => $header->toString(),
+                'Value' => $header->getBodyAsString(),
             ];
         }
 

src/Symfony/Component/Mailer/Bridge/Sendgrid/Tests/Transport/SendgridApiTransportTest.php

@@ -13,6 +13,7 @@ namespace Symfony\Component\Mailer\Bridge\Sendgrid\Tests\Transport;
 
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Mailer\Bridge\Sendgrid\Transport\SendgridApiTransport;
+use Symfony\Component\Mailer\Envelope;
 use Symfony\Component\Mime\Address;
 use Symfony\Component\Mime\Email;
 use Symfony\Contracts\HttpClient\HttpClientInterface;
@@ -149,4 +150,20 @@ class SendgridApiTransportTest extends TestCase
 
         $mailer->send($email);
     }
+
+    public function testCustomHeader()
+    {
+        $email = new Email();
+        $email->getHeaders()->addTextHeader('foo', 'bar');
+        $envelope = new Envelope(new Address('alice@system.com'), [new Address('bob@system.com')]);
+
+        $transport = new SendgridApiTransport('ACCESS_KEY');
+        $method = new \ReflectionMethod(SendgridApiTransport::class, 'getPayload');
+        $method->setAccessible(true);
+        $payload = $method->invoke($transport, $email, $envelope);
+
+        $this->assertArrayHasKey('headers', $payload);
+        $this->assertArrayHasKey('foo', $payload['headers']);
+        $this->assertEquals('bar', $payload['headers']['foo']);
+    }
 }

src/Symfony/Component/Mailer/Bridge/Sendgrid/Transport/SendgridApiTransport.php

@@ -104,7 +104,7 @@ class SendgridApiTransport extends AbstractApiTransport
                 continue;
             }
 
-            $payload['headers'][$name] = $header->toString();
+            $payload['headers'][$name] = $header->getBodyAsString();
         }
 
         return $payload;