mailpipe: fix _get_message_subject() doctest for "-ascii part".

mailpipe: fix _get_message_subject() doctest for "-ascii part".

In both Python 3.2.3 and 3.3 the doctest subject is encoded as

  '=?utf-8?q?unicode_part?= -ascii part'

In Python 3.3, this is now correctly decoded (I think) to retain the
space.  It seems to me that the encoded version should actually be:

  '=?utf-8?q?unicode_part?=-ascii part'

but I don't care enough to track that down ;).