[HttpFoundation] Do not set the default Content-Type based on the Acc…

…ept header

ErrorRenderer/SerializerErrorRenderer.php

@@ -12,6 +12,7 @@
 namespace Symfony\Component\ErrorHandler\ErrorRenderer;
 
 use Symfony\Component\ErrorHandler\Exception\FlattenException;
+use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\RequestStack;
 use Symfony\Component\Serializer\Exception\NotEncodableValueException;
 use Symfony\Component\Serializer\SerializerInterface;
@@ -30,6 +31,7 @@ class SerializerErrorRenderer implements ErrorRendererInterface
 
     /**
      * @param string|callable(FlattenException) $format The format as a string or a callable that should return it
+     *                                                  formats not supported by Request::getMimeTypes() should be given as mime types
      * @param bool|callable                     $debug  The debugging mode as a boolean or a callable that should return it
      */
     public function __construct(SerializerInterface $serializer, $format, ErrorRendererInterface $fallbackErrorRenderer = null, $debug = false)
@@ -57,11 +59,16 @@ public function render(\Throwable $exception): FlattenException
 
         try {
             $format = \is_string($this->format) ? $this->format : ($this->format)($flattenException);
+            $headers = [
+                'Content-Type' => Request::getMimeTypes($format)[0] ?? $format,
+                'Vary' => 'Accept',
+            ];
 
             return $flattenException->setAsString($this->serializer->serialize($flattenException, $format, [
                 'exception' => $exception,
                 'debug' => \is_bool($this->debug) ? $this->debug : ($this->debug)($exception),
-            ]));
+            ]))
+            ->setHeaders($flattenException->getHeaders() + $headers);
         } catch (NotEncodableValueException $e) {
             return $this->fallbackErrorRenderer->render($exception);
         }