Making bugfix in commit 07b2c6316a3cf623ca4ea4be8d680e46b6b8f9d4 to spree promo order decorator as well

https://github.com/spree/spree/issues/3612 and https://github.com/spree/spree/issues/3613 are being addressed by changes to the `orders_controller`. However, we need to also modify the `promo/app/controllers/spree/orders_controller_decorator.rb` to also include the `if @order.cart?` line, otherwise the older broken code in this decorator will override the fixed code in the core `orders_controller`.