[IMP] contract_variable_quantity: Recompute price when changing qty

If you have contract lines with automatic price and your pricelist
contains different prices per quantity, the price is not changed
when computing quantity.

This PR fixes this.

contract_variable_quantity/models/contract.py

@@ -38,6 +38,10 @@ class AccountAnalyticAccount(models.Model):
                 vals = {}
             else:
                 vals['quantity'] = qty
+                # Re-evaluate price with this new quantity
+                vals['price_unit'] = line.with_context(
+                    contract_line_qty=qty,
+                ).price_unit
         return vals