don't prefix default namespace elements with 'xmlns:'

I apologize, my PR #279 and its predecessor #173 didn't fully take into account recent code refactorings (it was a complete, production-tested solution on an older version of node-soap).  As a result, while the new test now passes and node-soap no longer throws exceptions on default namespace elements, they are incorrectly prefixed by 'xmlns:' in the generated XML. :grimacing:

This doesn't break any of node-soap's previously supported functionality, it only causes server errors when the server receives default namespace elements incorrectly prefixed by 'xmlns:'.  In other words, only the new default namespace feature introduced in PR #279 is partially broken.