fix #13590 - allow non-ASCII characters for passwords during LOGIN

Without that there is no way, that horde can be used together with
imapproxy, because imapproxy only supports the LOGIN mechanism and
if we don't support non-ASCII characters within a password, lots of
passwords won't be accepted.
Most imap servers (e.g. dovecot) accept non-ASCII characters
during LOGIN, which makes this change safe.