public function cacheUntil($expires)
    {
        if (!is_int($expires)) {
            $expires = strtotime($expires);
        }
        return $this
            ->withHeader('Pragma', 'public')
            ->withHeader('Cache-Control', 'maxage='.($expires - time()))
            ->withHeader('Expires', gmdate('D, d M Y H:i:s', $expires).' GMT');
    }