docs: def _prepare_request(self, url, method, headers, data):
          """Prepare HTTP request.
  
          :param str url: request URL.
          :param str method: request method.
          :param dict headers: request headers.
          :param object data: JSON-encodable object.
  
          :rtype: httpclient.HTTPRequest
  
          """
  
          request = httpclient.HTTPRequest(
              url=url, method=method, headers=headers, body=data,
              connect_timeout=self._connect_timeout,
              request_timeout=self._request_timeout,
              auth_username=self._username, auth_password=self._password,
              client_cert=self._client_cert, client_key=self._client_key,
              ca_certs=self._ca_certs, validate_cert=self._verify_cert)
  
          return request