[FIX] mail: specific message subscribe

`write` expects a `dict` for the `values` argument,
`part[record.id]` is a list of commands to add new
followers.

See the returned variable `specific` of the method
`_add_follower_command` in `mail_followers.py`.

Surrounding `part[record.id]` with a `dict` with
`message_follower_ids` has probably been
forgotten by oversight.

`gen` and `part[record.id]` have actualy the same
syntax, and the `write` done with these variables
should therefore be called the same way.

opw-669376

addons/mail/models/mail_thread.py

@@ -1807,7 +1807,7 @@ class MailThread(models.AbstractModel):
         gen, part = self.env['mail.followers']._add_follower_command(self._name, self.ids, partner_data, channel_data, force=force)
         self.sudo().write({'message_follower_ids': gen})
         for record in self.filtered(lambda self: self.id in part):
-            record.write(part[record.id])
+            record.write({'message_follower_ids': part[record.id]})
 
         self.invalidate_cache()
         return True