[FIX] bugs discovered by failing onwrite test case

[FIX] bugs discovered by failing onwrite test case

Following xmo@openerp.com-20130607120355-x3poxy2ar2bpqqvw:

* add_ids should not add ids which are already in the dataset, this
  leads to duplicates which the web client does not overly like

* methods which add or remove records should not manipulate
  dataset.ids as well as that's now automatic (on_write feature)

* record add should only insert the id in the dataset on non-false ids
  (e.g. list edition uses "pending" record with false id during
  creation, then sets the id it got from create() call)

bzr revid: xmo@openerp.com-20130607152326-2pja1kuwo0ropfuw