Response: Set header Content-Type for every response by default

refs #12161

library/Icinga/Web/Response.php

@@ -275,6 +275,8 @@ class Response extends Zend_Controller_Response_Http
                 $this->setRedirect($redirectUrl->getAbsoluteUrl());
             }
         }
+
+        $this->setHeader("Content-Type", $this->getContentType(), true);
     }
 
     /**

library/Icinga/Web/Response/JsonResponse.php

@@ -191,7 +191,7 @@ class JsonResponse extends Response
      */
     public function sendHeaders()
     {
-        $this->setHeader('Content-Type', 'application/json', true);
+        $this->setContentType('application/json');
         parent::sendHeaders();
     }