Use the amount_second_currency from the line and not from the invoice

issue10916
review389191002
(grafted from d7557a33425a2bbce37dd3f8a3eaf41d75db3ccc)

--HG--
branch : 6.0

account.py

@@ -156,7 +156,7 @@ class Invoice(metaclass=PoolMeta):
             currency = line.second_currency
             if currency:
                 line.amount_second_currency = currency.cash_round(
-                    self.amount_second_currency)
+                    line.amount_second_currency)
         return line
 
     def get_move(self):