Update order state when moving back through checkout step

Some important logic happens after the order transition from one state
to another. In particular, voucher are recalculated. To fix any
inconsistency, we make sure the order is the state matching the checkout
step we are on.

app/controllers/split_checkout_controller.rb

@@ -24,6 +24,8 @@ class SplitCheckoutController < ::BaseController
 
   def edit
     redirect_to_step_based_on_order unless params[:step]
+
+    update_order_state if params[:step]
     check_step if params[:step]
 
     return if available_shipping_methods.any?