The mock server implementation does not exactly match the actual orm
implementation. It is not really a big deal, but it makes the test suite
less useful.
In particular, the onchange call should return a value for all fields
when we are creating a new record. Also, the default get implementation
was not exactly correct.
The motivation for this change is to prepare the future relational model
refactoring.
closesodoo/odoo#117434
Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>