Re diffs, noted for the future. 
Re tests:

# py3k-devel/Lib/test % grep -r getallmatchingheaders *

... Returns nothing, so not only does the email package need a test for 
this but so does http.client. 

Incidentally test_mailbox.py has a test for the proposed alternative - 
get_all(), which I noted above. That's another good reason for ridding 
the world of getallmatchingheaders() or at least simply calling 
get_all() from within getallmatchingheaders() if compatibility is a 
legitimate concern.