Make addresses case insensitive for the purpose of constructing replies.

The domain is alway case insensitive, but in principle the username is
case sensitive.  Few systems actually enforce this so I think a good
default is to treat the entire address as case insensitive, it will
eliminate a lot of superfluous self-addressed messages and reply from
the correct address in these cases.

Signed-off-by: Jed Brown <jed@59A2.org>