protected function sendNotModified(HTTPRequest $request, HTTPResponse $response)
    {
        // 304 is invalid for destructive requests
        if (in_array($request->httpMethod(), ['POST', 'DELETE', 'PUT'])) {
            $response->setStatusCode(412);
        } else {
            $response->setStatusCode(304);
        }
        $response->setBody('');
        return $response;
    }