/*
   * WS-Invoke
   *
   * invoke: Invoke a resource function
   *
   * call-seq:
   *   client.invoke(options, uri, "method-name") -> XmlDoc
   *   client.invoke(options, uri, "method-name", xml_doc) -> XmlDoc
   *
   */
  WsXmlDocH invoke( client_opt_t *options, const char *resource_uri, const char *method, WsXmlDocH data = NULL) {