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

        /**
         * the "W/" prefix is necessary to qualify it as a "weak" Etag.
         * only then a proxy like nginx will leave the tag alone because a strong cannot
         * match if gzip is applied.
         */
        $response->headers->set('ETag', 'W/'.sha1($response->getContent()));
    }