code: @Override
      public int addDefaultAttribute(String localName, String uri, String prefix,
                                     String value) throws XMLStreamException
      {
          return mAttrCollector.addDefaultAttribute(localName, uri, prefix, value);
      }
docs: Method called by actual validator instances when attributes with
  default values have no explicit values for the element; if so,
  default value needs to be added as if it was parsed from the
  element.