private function appendLastModified($timestamp, $maxAge)
    {
        $headers = array(
            'Cache-Control' => sprintf('public, max-age=%s', $maxAge),
            'Last-Modified' => gmdate('D, j M Y H:i:s', $timestamp).' GMT'
        );

        $this->headerBag->setStatusCode(200)
                        ->appendPairs($headers);
    }