public function head($url, array $data = array(), array $extra = array())
    {
        if (!empty($data)) {
            $url = $url . '?' . http_build_query($data);
        }

        $curl = new Curl(array(
            CURLOPT_URL => $url,
            CURLOPT_RETURNTRANSFER => true,
            CURLOPT_NOBODY => true,
            CURLOPT_POSTFIELDS => http_build_query($data)
        ), $extra);

        return $curl->exec();
    }