0.4.x branch: Avoid alias method chain twice

Dave,

I had a situation today where I ran across a bunch of stack too deep errors in the Rails app i'm working on.

SystemStackError: stack level too deep    
     stonepath (0.4.1) [v] lib/stonepath/work_item.rb:41:in `to_xml_without_events'
     stonepath (0.4.1) [v] lib/stonepath/work_item.rb:32:in `to_xml'

For some reason it appears that the alias_method_chain for :to_xml is getting called twice. This patch adds a check: only do alias_method_chain if we have NOT done it before.