simple_current_order only gets incomplete orders

This currently causes an issue with better_spree_paypal_express, and possibly
other extensions.

While Spree sets the session's order_id to nil when an order is complete,
better_spree_paypal_express does not.  Therefore, a completed order can be
retrieved as the simple_current_order.

It makes sense to replicate the logic of current_order (which does not allow
completed orders to be returned) here.