update to support the latest xmlbuilder

I'm working on packaging node-xmlrpc for Fedora so I need to get it working with the latest versions of its dependencies.

Looks like xmlbuilder changed API quite a while back. Hopefully this is the only change, there is unfortunately no changelog or release notes for xmlbuilder :disappointed: 
Thanks @patricklodder quite right, that was a silly mistake. I guess one difference between v1.2.0 and master is that there is better test coverage :-) Will update the patch.