added some unit tests for message parsing

added some unit tests for message parsing

excerpts from original message from Ismo Puustinen <ismo@iki.fi>:
I wrote one new unit test. The difference to previous tests is that
this test actually currently fails: for some reason, dividing a
message body into chunks doesn't succeed. The message in question
is an actual email message sent to vim-mac mailing list; only
the names and email addresses have been obscured. I selected the
message for the tests because it kept crashing my sup-sync. :-)