feature #19552 [HttpFoundation] Add named constructor on JsonResponse (tyx)

This PR was merged into the 3.2-dev branch.

Discussion
----------

[HttpFoundation] Add named constructor on JsonResponse

| Q             | A
| ------------- | ---
| Branch?       | "master"
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

To make easier construction with raw json. It is a simple addition but having something like :
```php
return new JsonResponse($content, Response::HTTP_OK, [], true);
```

and
```php
return new JsonResponse($content, Response::HTTP_OK, []);
```

is not very obvious to get the difference without going to read the JsonResponse constructor.

Commits
-------

6d5a65d Add named constructor on JsonResponse

src/Symfony/Component/HttpFoundation/JsonResponse.php

@@ -58,6 +58,14 @@ class JsonResponse extends Response
         return new static($data, $status, $headers);
     }
 
+    /**
+     * Make easier the creation of JsonResponse from raw json.
+     */
+    public static function fromJsonString($data = null, $status = 200, $headers = array())
+    {
+        return new static($data, $status, $headers, true);
+    }
+
     /**
      * Sets the JSONP callback.
      *

src/Symfony/Component/HttpFoundation/Tests/JsonResponseTest.php

@@ -198,6 +198,12 @@ class JsonResponseTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals('{"0":{"0":1,"1":2,"2":3}}', $response->getContent());
     }
 
+    public function testItAcceptsJsonAsString()
+    {
+        $response = JsonResponse::fromJsonString('{"foo":"bar"}');
+        $this->assertSame('{"foo":"bar"}', $response->getContent());
+    }
+
     /**
      * @expectedException \InvalidArgumentException
      */