public function getCacheHeaders(ConcretePage $c)
    {
        $lifetime = $c->getCollectionFullPageCachingLifetimeValue();
        $expires = gmdate('D, d M Y H:i:s', time() + $lifetime) . ' GMT';

        $headers = [
            'Pragma' => 'public',
            'Cache-Control' => 'max-age=' . $lifetime . ',s-maxage=' . $lifetime,
            'Expires' => $expires,
        ];

        return $headers;
    }