[FIX] models: default_get() shall not return a dict as a many2one value

When a new record is returned as the value for a many2one on a new record, the
method Many2one.convert_to_write() now returns a NewID, and default_get() then
discards that value from its result. This makes it consistent with its former
behavior.

Manual rebase of #1547
[FIX] purchase: fix test merge_order (check on date_order was not correct)