TypeError thrown when None returned by Caiwireless

The Caiwireless detector returns None if the message is multipart/mixed but no failures were found.

> ipython
>>> from flufl.bounce import scan_message
>>> from email import message_from_file
>>> scan_message(message_from_file(open('flufl/bounce/tests/data/simple_01.txt')))
No handlers could be found for logger "flufl.bounce"
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
/home/paul/download/flufl.bounce/<ipython-input-3-2eb556669605> in <module>()
----> 1 scan_message(message_from_file(open('flufl/bounce/tests/data/simple_01.txt')))

/home/paul/download/flufl.bounce/flufl/bounce/_scan.pyc in scan_message(msg)
     68 log.info('Running detector: {0}'.format(detector_class))
     69 try:
---> 70 temporary, permanent = detector_class().process(msg)
     71 except Exception:
     72 log.exception('Exception in detector: {0}'.format(detector_class))

TypeError: 'NoneType' object is not iterable