[IMP] mail: added headers field on mail.mail, allowing to store headers data when sending notification emails.
Merge pull request #691 from odoo-dev/master-mail-headers-tde

[IMP] mail: added headers field on mail.mail, allowing to store headers data when sending notification emails. This commit in master improves the commit 839815fdd44742663742b3506329d5b46cb3e3f7 (saas-5) that was using computed but not stored values.
Merge branch 'master' of https://github.com/odoo/odoo into master-linux64