[FIX] mail.thread: avoid overwriting thread_id/model with foreign values when reply-to hostname does not match + fix tests

This improve previous commit by making sure we
never consider the thread_id/model values in the
In-Reply-To/References header if the host name
did not match.
Also fixes the tests that were using the
6.1 compatibility mode to post in a mail group
thread instead of specifying the right
message-id.