protected function _generateProxyRequestHeaders($as_string = FALSE)
    {
        $headers = array();
        $headers[] = 'Content-Type: ' . $this->_requestContentType;

        if ($this->_requestAuthorization) {
            $headers[] = 'Authorization: ' . $this->_requestAuthorization;
        }

        if ($this->_requestAccept) {
            $headers[] = 'Accept: ' . $this->_requestAccept;
        }

        if($as_string)
        {
            $data = "";
            foreach($headers as $value)
                if($value)
                    $data .= "$value\n";

            $headers = $data;
        }

        return $headers;
    }