[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