public function setHeaders(array $headers, $overwrite_existing = false)
    {
        if ($overwrite_existing) {
            $this->curl_opts[CURLOPT_HTTPHEADER] = $headers;
        } else {
            $this->curl_opts[CURLOPT_HTTPHEADER] = array_merge(
                $this->curl_opts[CURLOPT_HTTPHEADER],
                $headers
            );
        }
    }