[FIX] product_visible_discount: use on_change result for product value

At the end of the onchange call product_id_change, the uom may have changed (e.g. if product in different category).
To compute the quantity, we need to use the new uom and not the first one (that may be Unit, default value)
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-linux64