private function init()
    {
        $this->curl = curl_init();
        $this->setUserAgent(self::USER_AGENT);
        $this->setOpt(CURLINFO_HEADER_OUT, true);
        $this->setOpt(CURLOPT_HEADER, false);
        $this->setOpt(CURLOPT_RETURNTRANSFER, true);
        $this->setOpt(CURLOPT_HEADERFUNCTION, array($this, 'addResponseHeaderLine'));
        return $this;
    }