[FIX] account: Computation of a fixed amount tax

When an order with a fixed amount tax was returned the sign of the tax
was wrong because amount_base was negative and quantity was negative too,
then the tax was positive due to math.copysign(self.amount, base_amount) * abs(quantity)

The sign of the base amount already takes into account the the sign
of the quantity.

opw:682407

addons/account/models/account.py

@@ -613,7 +613,9 @@ class AccountTax(models.Model):
         """
         self.ensure_one()
         if self.amount_type == 'fixed':
-            return math.copysign(self.amount, base_amount) * abs(quantity)
+            # Use copysign to take into account the sign of the base amount which includes the sign
+            # of the quantity and the sign of the price_unit
+            return math.copysign(quantity, base_amount) * self.amount
         if (self.amount_type == 'percent' and not self.price_include) or (self.amount_type == 'division' and self.price_include):
             return base_amount * self.amount / 100
         if self.amount_type == 'percent' and self.price_include: