fix for https://github.com/jasonrbriggs/stomp.py/issues/99 (on_before_message throwing a 'not iterable' error)
fix for https://github.com/jasonrbriggs/stomp.py/issues/99 (on_before_message throwing a 'not iterable' error)

jasonrbriggs/stomp.py,jasonrbriggs/stomp.py