Purpose of this commit is to add tests to check current behavior of composer
when he is updated with Falsy values :
* template set, with void value for some fields that may reset or not
existing composer values;
* template being set to False, check what happens with composer values
(kept or reset);
This may change in a near future but at least it will be documented.
Task-2710804 (Mail: Clean MailThread Posting API)
closes odoo/odoo#108320
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>