Only check if expected_quantity has changed

Since rev 7166c43434f7, the quantity field is no more filled.

issue8433
review285371002

inventory.py

@@ -460,7 +460,7 @@ class InventoryLine(ModelSQL, ModelView):
         '''
         values = {}
         # if nothing changed, no update
-        if self.quantity == self.expected_quantity == quantity:
+        if self.expected_quantity == quantity:
             return values
         values['expected_quantity'] = quantity
         return values