Send the correct HTTP response for unsupported mimetype.

core/lib/Drupal/Core/EventSubscriber/HtmlSubscriber.php

@@ -89,7 +89,7 @@ public function onView(GetResponseEvent $event) {
       $event->setResponse(new Response(drupal_render_page($page_callback_result)));
     }
     else {
-      $event->setResponse(new Response('Requested MIME type not supported', 400));
+      $event->setResponse(new Response('Unsupported Media Type', 415));
     }
   }