[FIX] web: handle 'domain' key returned by onchanges

Onchange RPCs return an object that may contain a 'domain' key.
When it does, its value is an object whose keys are field names and
values are the new domain for the corresponding field.

Before this rev., the 'domain' key was totally ignored by the
BasicModel. This feature is used for example in the account.payment
form view (go to an open invoice, click on 'Register payment'): the
domain of field payment_method_id is updated by the onchanges (e.g.
when changing the journal_id). As the domain was ignored, it always
displayed all possible payment methods.