Defend against certain malformed messages.

Accommodate https://bugs.python.org/issue27321 by overriding as_string() in
mailman.email.message.Message to catch the KeyError, get the message
as_bytes() and convert it to a string.  This obviates !197 as a fix for #256
so that is reverted.

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