private function addRequestHeaders(&$curlOptions) {
        // cURL HTTP headers
        $headers = $this->getOption(self::HEADERS_KEY, array());
        // Turn off the Expect header to stop HTTP 100 Continue responses.
        // Response parsing was not handling these headers.
        $curlOptions[CURLOPT_HTTPHEADER][] = "Expect:";
        if (0 < count($headers)) {
            $curlOptions[CURLOPT_HTTPHEADER] = array();
            foreach ($headers as $key => $value) {
                $curlOptions[CURLOPT_HTTPHEADER][] = sprintf("%s:%s", $key, $value);
            }
        }
    }