protected function addHeaders()
    {
        $userHeaders = array_map(function ($key, $value) {
            // format header like this 'x-header: value'
            return sprintf('%s: %s', $key, $value);
        }, array_keys($this->payload['headers']), $this->payload['headers']);

        $headers = array_merge($this->defaultHeaders, $userHeaders);

        $this->addCurlOption(CURLOPT_HTTPHEADER, $headers);
    }