Issue

	- Accounting & Purchase
	- Create a PO for wood corner
	- Create a bill for wood corner, add a ref
	  and auto complete with the previous PO

	The ref is replaced by the PO's one

Cause

	In purchase account_invoice _onchange_purchase_auto_complete
	we update all the bill values with the PO values including
	the PO ref (empty or not)

Solution

	Exclude the ref of fields to be updated

OPW-2313418

closes odoo/odoo#55560

X-original-commit: ae0c3b600403a21a196e51ce88fa6b25990f2b64
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>