[FIX] mail_mail: computation of reply_to after create to avoid
interfering with automatic message_id creation of mail_message
[FIX] mail_thread: fixed issues with private messages through mailgateway
(wrong route checking, variable erasing)
[TEST] mail: added test for the second bug
[CLEAN] mail: tests: cleaned test_mail_gateway file about indentation
[MERGE] [FIX] mail_mail: computation of reply_to after create to avoid
interfering with automatic message_id creation of mail_message
[FIX] mail_thread: fixed issues with private messages through mailgateway
(wrong route checking, variable erasing)
[TEST] mail: added test for private discussions
[CLEAN] mail: tests: cleaned test_mail_gateway file about indentation