[FIX] hr_contract: count the number of contracts of an employee as superuser_id as this field is required in the form view. However no data is displayed, and no data about the contracts are leaked, so no security holes.
[FIX] fields: convert_to_cache() on *2many fields must take record's current value

The existing code was buggy when writing on *2many fields with a list of
commands: the value was converted for the cache, but taking an empty recordset
as the current value of the field.