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.