public static function getDirective(MessageInterface $message, $part)
    {
        $parts = AbstractMessage::parseHeader($message, 'Cache-Control');

        foreach ($parts as $line) {
            if (isset($line[$part])) {
                return $line[$part];
            } elseif (in_array($part, $line)) {
                return true;
            }
        }

        return null;
    }