When a message is processed through the mail gateway, the partner should
never be set as the assignee of that task.
This commit allow to correctly let pass `{'default_user_ids': False}`
context from `message_new()` down to `create()`
closes odoo/odoo#110124
X-original-commit: 08e24d683bf19a77982a7ffba6c64b5fc6fdf85a
Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>