[FIX] mail: have signature in chatter by default

The signature is shown by default when sending a mail and in particular
case it is not shown.

In c8d283a592 there was a small regression that in some instance the
signature that was shown before is not shown anymore. This is because
the default in those instance is to not show signature.

eg. a message sent in a chatter would not have signature in sent emails

With this commit, the previous behavior is reintroduced.

opw-1903001
opw-1896262
closes #29088

addons/mail/models/mail_thread.py

@@ -1926,7 +1926,7 @@ class MailThread(models.AbstractModel):
     def message_post(self, body='', subject=None,
                      message_type='notification', subtype=None,
                      parent_id=False, attachments=None,
-                     notif_layout=False, add_sign=False, model_description=False,
+                     notif_layout=False, add_sign=True, model_description=False,
                      mail_auto_delete=True, **kwargs):
         """ Post a new message in an existing thread, returning the new
             mail.message ID.