JsonResponse: Set content type at initialization and not at sendHeaders

refs #12161

library/Icinga/Web/Response/JsonResponse.php

@@ -67,6 +67,13 @@ class JsonResponse extends Response
      */
     protected $successData;
 
+    /**
+     * Content type of this response
+     *
+     * @var string
+     */
+    protected $contentType = 'application/json';
+
     /**
      * Get the JSON encoding options
      *
@@ -186,15 +193,6 @@ class JsonResponse extends Response
         echo json_encode($body, $this->getEncodingOptions());
     }
 
-    /**
-     * {@inheritdoc}
-     */
-    public function sendHeaders()
-    {
-        $this->setContentType('application/json');
-        parent::sendHeaders();
-    }
-
     /**
      * {@inheritdoc}
      */