public function onKernelResponse(FilterResponseEvent $event)
    {
        $response = $event->getResponse();
        $request = $event->getRequest();

        $ifNoneMatch = $request->headers->get('if-none-match');
        $etag = $response->headers->get('ETag', 'empty');

        if ($ifNoneMatch === $etag) {
            $response->setStatusCode(304);
            $response->setContent(null);
        }
    }