public function withNotModified()
    {
        $this->withStatus(static::HTTP_NOT_MODIFIED);
        $this->getBody()->write('');

        // remove headers that MUST NOT be included with 304 Not Modified responses
        foreach ([
                     'Allow',
                     'Content-Encoding',
                     'Content-Language',
                     'Content-Length',
                     'Content-MD5',
                     'Content-Type',
                     'Last-Modified'] as $header) {
            $this->withoutHeader($header);
        }

        return $this;
    }