[FIX] purchase: warning uom category different only if the changed field is product_uom itself.

Changing product_id with a different unit category does not trigger the uom category change warning
[MERGE] Forward-port of latest 7.0 bugfixes, up to rev. 9678 rev-id: dle@openerp.com-20131206121552-xre6jj34cpfuldoa