[FIX] sale: taxes on report

- Create a tax using the tax template 'GST + QST for sales'
- Create a SO with the following lines:
  Product 1 100 Units @ 0.1
  Product 2 2000 Units @ 10.334
  Product 3 2000 Units @ 10.334
  Product 4 5000 Units @ 0.1
  Product 5 1500 Units @ 0.1
- The totals are:
  Amount without taxes: 41996.00
  Taxes: 6288.90

When printed, the quotation gives:
  Amount without taxes: 41996.00
  Taxes: 6286.52

This is due to the method `_get_tax_amount_by_group`, which is using
`line.price_reduce` for tax computation. Since it is a Monetary field,
it is automatically rounded to the currency precision, while it should
be rounded with the same precision as `price_unit`.

To avoid breaking overrides, changing the field type is done in master
branch.

Introduced in 99743cc4aa

opw-752487

addons/sale/models/sale.py

@@ -506,12 +506,14 @@ class SaleOrder(models.Model):
             for tax in line.tax_id:
                 group = tax.tax_group_id
                 res.setdefault(group, 0.0)
-                taxes = tax.compute_all(line.price_reduce + base_tax, quantity=line.product_uom_qty,
+                # FORWARD-PORT UP TO SAAS-17
+                price_reduce = line.price_unit * (1.0 - line.discount / 100.0)
+                taxes = tax.compute_all(price_reduce + base_tax, quantity=line.product_uom_qty,
                                          product=line.product_id, partner=self.partner_shipping_id)['taxes']
                 for t in taxes:
                     res[group] += t['amount']
                 if tax.include_base_amount:
-                    base_tax += tax.compute_all(line.price_reduce + base_tax, quantity=1, product=line.product_id,
+                    base_tax += tax.compute_all(price_reduce + base_tax, quantity=1, product=line.product_id,
                                                 partner=self.partner_shipping_id)['taxes'][0]['amount']
         res = sorted(res.items(), key=lambda l: l[0].sequence)
         res = map(lambda l: (l[0].name, l[1]), res)