public function etag($hash = null, $weak = false)
    {
        deprecationWarning(
            'Response::etag() is deprecated. ' .
            'Use withEtag() or getHeaderLine("Etag") instead.'
        );

        if ($hash !== null) {
            $this->_setHeader('Etag', sprintf('%s"%s"', $weak ? 'W/' : null, $hash));
        }

        if ($this->hasHeader('Etag')) {
            return $this->getHeaderLine('Etag');
        }

        return null;
    }