Before this patch, when searching with the "equals to" operator in any
x2many field, the searched parameter was always `undefined`.
The problem was that the underlying field manager implementation was
treating those fields as x2many, while the widget used was the `one2many`
one.
This patch simply mocks the underlying fake record to make think that
any relational field is always a `one2many`. This sets all pieces in
place and makes the field manager work as expected, and thus you can
search as expected too.
Jairo Llopis
7cc961fb7b