[FIX] sale_coupon: default invoicing policy

The default invoicing policy should always be by ordered quantity.
Otherwise, if the user changes the default invoicing policy to
"delivered quantity", coupon won't be applied to the invoice.

opw-749632

models/sale_coupon_program.py

@@ -80,7 +80,8 @@ class SaleCouponProgram(models.Model):
                 'taxes_id': False,
                 'supplier_taxes_id': False,
                 'sale_ok': False,
-                'purchase_ok': False
+                'purchase_ok': False,
+                'invoice_policy': 'order',
             })
             program.write({'discount_line_product_id': discount_line_product_id.id})
         return program