When creating a new record, the client calls `default_get()`, completes
the returned values with `False`, and calls `onchange()` to apply the
onchange to the defaults.
Optimize the double round-trip by integrating `default_get()` inside
`onchange()` for the first call. The method is called with an empty
list of fields, and usually no field values, except for records in a
one2many field. In this case, `onchange()` does both steps above.
Task 2261084
Raphael Collet
488e334fc8