/*
   * get node attribute by index or name
   *
   * call-seq:
   *  node.attr(1) -> XmlAttr
   *  node.attr("name") -> XmlAttr
   *  node.attr("name", "namespace") -> XmlAttr
   *
   */
  WsXmlAttrH attr(VALUE index = Qnil, VALUE namespace = Qnil) {