Fix return type for mails with override msgcls

* src/mapihelp.cpp (mapi_change_message_class): Improve logging
in override case.
(mapi_change_message_class): Ensure r_type is set correctly.

This fixes some cases where the message class was already set.
Apparently this happens on Exchange / Exchange Online as I
was only able to reproduce the problem there. It might happen
in other set ups where the server already stores our GpgOL
message class.

• GnuPG-Bug-Id: T3537