Missing parent relation when _parent_ is used in depends
I found this buggy behavior on a proteus script that create sale line and set the sale field on it.
After the call to on_change_quantity, the sale field has been changed to None.
This is because the on_change_quantity depends on '_parent_*' fields but not on 'sale'. So proteus does not sent the sale (nor the parent because it is not in a One2Many).
I think the behavior of proteus is correct. It is the on_change that should always depends on the relation field if it depends on the _parent_.
I propose to add a generic test for that.