clone(): A new method for creating a clone of this generator (for
recursive generation).
_dispatch(): If the message object doesn't have a Content-Type:
header, check its default type instead of assuming it's text/plain.
This makes for correct generation of message/rfc822 containers.
_handle_multipart(): We can get rid of the isdigest kludge. Just
print the message as normal and everything will work out correctly.
_handle_mulitpart_digest(): We don't need this anymore either.