_handle_multipart(): Fixes for SF bug #531966. Specifically two

_handle_multipart(): Fixes for SF bug #531966.  Specifically two
situations are handled now: a multipart/* containing no payload
(i.e. never set), and a multipart/* containing a scalar payload
(i.e. Message.add_payload() having been called exactly once, not
passing in a sequence object).

_make_boundary(): Fixed bogus cut-n-paste error (self as first arg).

I will merge these changes into the standalone email package and
Python 2.3 separately.