Merge pull request #288 from odoo-dev/saas-4-mass_mailing_fixes-tde

[FIX] mass_mailing: fixes
- backport (+ cleaning) of eb22d202 (saas-5): mail_thread: routing: instead of exclusive routing heuristics, use each case as a fallback of the previous.
- better fix for dd36a0e5 (saas-5): mail_thread: routing: fixed replies always choosen even when replying to emails with a specified reply_to (using ref_match in the algorithm)
- backport of d6a2ae64 (saas-5): avoid evaluating a False /  None domain
- fixed keeping the original message for routing, only when choosing to reply in the original thread (notification=True)
- mail_thread: routing: fixed replies always chosen even when replying to emails with a specified reply_to (using ref_match in the algorithm)