improve override of /api/orders/show output

spree_hub_connector was dropping some things that spree had included, like [display_item_total](https://github.com/spree/spree/blob/b0d5242/api/app/views/spree/api/orders/order.v1.rabl#L2)
so instead of overriding the view this just adds extra attributes to "order_attributes".