[FIX] web: fix one2many default get

The one2many default get code used to meddle with basic_model's res_ids
manually, which ended up desynchronizing the data and res_ids fields
of the list representation of the one2many values.

This commit solves the issue by using the proper mechanisms of the
basic_model, that is creating a proper datapoint for those ids.

Steps to reproduce the original issue:
- Install sale_mamangement, mrp and purchase
- Create a product
- In the Inventory tab, in Routes, uncheck Buy
- Check and then uncheck back any other route than Buy
- Get a traceback because we are looking for a record with a given
  res_id in the list data but since they are desynchronized, data
  is empty at that point even though res_ids is not.