[FIX] pos: do not mix create and validation date
[FIX] pos: do not mix create and validation date

The validation_date was using the creation_date value in the follwing scenario:
1. create a new order with a few lines (creation_date=t1)
2. reload the page, `init_from_JSON` uses validation_date=t1
3. validate the order at t2, ignored due to already stored value

creation_date must be kept for tracablity reasons (e.g. pos_blackbox_be)

Fixes https://www.odoo.com/groups/59/20542134

bplancher/odoo,laslabs/odoo,hip-odoo/odoo,ygol/odoo,Elico-Corp/odoo_OCB,hip-odoo/odoo,bplancher/odoo,storm-computers/odoo,hip-odoo/odoo,microcom/odoo,laslabs/odoo,ygol/odoo,dfang/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,storm-computers/odoo,microcom/odoo,storm-computers/odoo,microcom/odoo,hip-odoo/odoo,Elico-Corp/odoo_OCB,hip-odoo/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,bplancher/odoo,hip-odoo/odoo,microcom/odoo,microcom/odoo,storm-computers/odoo,ygol/odoo,dfang/odoo,ygol/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,ygol/odoo,ygol/odoo,dfang/odoo,laslabs/odoo,laslabs/odoo,dfang/odoo,bplancher/odoo,microcom/odoo,storm-computers/odoo,dfang/odoo,laslabs/odoo,ygol/odoo,storm-computers/odoo,dfang/odoo