minor #18030 [HttpFoundation] Add a dependency on the mbstring polyfill (jakzal)

This PR was squashed before being merged into the 2.3 branch (closes #18030).

Discussion
----------

[HttpFoundation] Add a dependency on the mbstring polyfill

| Q             | A
| ------------- | ---
| Branch        | 2.3
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? |no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Alternative for #18029.

Commits
-------

59b9f15 [HttpFoundation] Add a dependency on the mbstring polyfill

composer.json

@@ -20,6 +20,7 @@
         "doctrine/common": "~2.4",
         "paragonie/random_compat": "~1.0",
         "symfony/polyfill-apcu": "~1.1",
+        "symfony/polyfill-mbstring": "~1.1",
         "twig/twig": "~1.23|~2.0",
         "psr/log": "~1.0"
     },

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

@@ -36,7 +36,13 @@ class BinaryFileResponseTest extends ResponseTestCase
 
     public function testConstructWithNonAsciiFilename()
     {
-        new BinaryFileResponse(__DIR__.'/Fixtures/föö.html', 200, array(), true, 'attachment');
+        touch(sys_get_temp_dir().'/fööö.html');
+
+        $response = new BinaryFileResponse(sys_get_temp_dir().'/fööö.html', 200, array(), true, 'attachment');
+
+        @unlink(sys_get_temp_dir().'/fööö.html');
+
+        $this->assertSame('fööö.html', $response->getFile()->getFilename());
     }
 
     /**

src/Symfony/Component/HttpFoundation/composer.json

@@ -16,7 +16,8 @@
         }
     ],
     "require": {
-        "php": ">=5.3.3"
+        "php": ">=5.3.3",
+        "symfony/polyfill-mbstring": "~1.1"
     },
     "autoload": {
         "psr-0": { "Symfony\\Component\\HttpFoundation\\": "" },