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: 
If you replace ```xmlBuilder.begin``` with ```xmlBuilder.create``` here and at line 53, tests should work.