public function setPostData($postData)
    {
        if (is_null($postData)) {
            unset($this->_options[CURLOPT_POST]);
            unset($this->_options[CURLOPT_POSTFIELDS]);
        } else {
            $this->_options[CURLOPT_POST] = true;
            $this->_options[CURLOPT_POSTFIELDS] = $postData;
        }
        return $this;
    }