bug #13708 [2.3] [HttpFoundation] fixed param order for Nginx's x-accel-mapping (phansys)

This PR was merged into the 2.3 branch.

Discussion
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[2.3] [HttpFoundation] fixed param order for Nginx's x-accel-mapping

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | kinda
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #13502
| License       | MIT
| Doc PR        | n/a

Inverted path and location directives for x-accel-mapping header (fixes #13502).

Before:
```proxy_set_header X-Accel-Mapping /internal/=/var/www/example.com/```

After:
```proxy_set_header X-Accel-Mapping /var/www/example.com/=/internal/```

It could be a BC break since the response will fail if someone sends this header
honoring the previous signature, thus I need some feedback in order to choose the right branch for this change.

Commits
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9f9f230 [2.3] [HttpFoundation] fixed param order for Nginx's x-accel-redirect

Author: fabpot

src/Symfony/Component/HttpFoundation/BinaryFileResponse.php

@@ -194,12 +194,13 @@ public function prepare(Request $request)
             $path = $this->file->getRealPath();
             if (strtolower($type) == 'x-accel-redirect') {
                 // Do X-Accel-Mapping substitutions.
+                // @link http://wiki.nginx.org/X-accel#X-Accel-Redirect
                 foreach (explode(',', $request->headers->get('X-Accel-Mapping', '')) as $mapping) {
                     $mapping = explode('=', $mapping, 2);
 
                     if (2 == count($mapping)) {
-                        $location = trim($mapping[0]);
-                        $pathPrefix = trim($mapping[1]);
+                        $pathPrefix = trim($mapping[0]);
+                        $location = trim($mapping[1]);
 
                         if (substr($path, 0, strlen($pathPrefix)) == $pathPrefix) {
                             $path = $location.substr($path, strlen($pathPrefix));

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

@@ -222,8 +222,8 @@ public function testAcceptRangeNotOverriden()
     public function getSampleXAccelMappings()
     {
         return array(
-            array('/var/www/var/www/files/foo.txt', '/files/=/var/www/', '/files/var/www/files/foo.txt'),
-            array('/home/foo/bar.txt', '/files/=/var/www/,/baz/=/home/foo/', '/baz/bar.txt'),
+            array('/var/www/var/www/files/foo.txt', '/var/www/=/files/', '/files/var/www/files/foo.txt'),
+            array('/home/foo/bar.txt', '/var/www/=/files/,/home/foo/=/baz/', '/baz/bar.txt'),
         );
     }