Defend against certain malformed messages.

Open username-removed-107958 requested to merge msapiro/mailman:as_string into master

Fixes #406 Fixes #256 (closed)

Accommodate by overriding as_string() in to catch the KeyError, get the message as_bytes() and convert it to a string. This obviates !197 (closed) as a fix for #256 (closed) so that is reverted.

Also fix mailman.mta.connection.Connection.sendmail to ensure msgtext is pure ascii.