Make variantStock.on_demand= save the variant so that it doesn't impact on_hand that is already saving the variant with a lock

app/models/concerns/variant_stock.rb

@@ -105,6 +105,7 @@ module VariantStock
     # unknown reasons, it does not pass the test.
     stock_items.each do |item|
       item.backorderable = new_value
+      item.save
     end
   end