Update file ResponseSubscriber.php

MR related to Update JsonIncludeSubscriber

src/EventSubscriber/ResponseSubscriber.php

@@ -101,6 +101,7 @@ class ResponseSubscriber implements EventSubscriberInterface {
       if ($need_parse) {
         $content = $response->getContent();
         if (strpos($content, '{"jsonapi"') === 0) {
+          $response->headers->set('Content-Type', 'application/json');
           $this->jsonapiInclude->parse($response);
         }
       }