[HttpFoundation] fix not sending Content-Type header for 204 responses

src/Symfony/Component/HttpFoundation/Response.php

@@ -267,10 +267,12 @@ public function prepare(Request $request)
             $this->setContent(null);
             $headers->remove('Content-Type');
             $headers->remove('Content-Length');
+            // prevent PHP from sending the Content-Type header based on default_mimetype
+            ini_set('default_mimetype', '');
         } else {
             // Content-type based on the Request
             if (!$headers->has('Content-Type')) {
-                $format = $request->getPreferredFormat();
+                $format = $request->getPreferredFormat(null);
                 if (null !== $format && $mimeType = $request->getMimeType($format)) {
                     $headers->set('Content-Type', $mimeType);
                 }

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

@@ -461,18 +461,10 @@ public function testSetVary()
 
     public function testDefaultContentType()
     {
-        $headerMock = $this->getMockBuilder('Symfony\Component\HttpFoundation\ResponseHeaderBag')->setMethods(['set'])->getMock();
-        $headerMock->expects($this->at(0))
-            ->method('set')
-            ->with('Content-Type', 'text/html');
-        $headerMock->expects($this->at(1))
-            ->method('set')
-            ->with('Content-Type', 'text/html; charset=UTF-8');
-
         $response = new Response('foo');
-        $response->headers = $headerMock;
-
         $response->prepare(new Request());
+
+        $this->assertSame('text/html; charset=UTF-8', $response->headers->get('Content-Type'));
     }
 
     public function testContentTypeCharset()