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.

spec/controllers/split_checkout_controller_spec.rb

@@ -67,6 +67,42 @@ describe SplitCheckoutController, type: :controller do
           expect(response).to redirect_to checkout_step_path(:payment)
         end
       end
+
+      context "when order state is 'confirmation'" do
+        before do
+          order.update!(state: "confirmation")
+        end
+
+        context "when loading payment step" do
+          it "updates the order state to payment" do
+            get :edit, params: { step: "payment" }
+
+            expect(order.reload.state).to eq("payment")
+          end
+        end
+
+        context "when loading address step" do
+          it "updates the order state to address" do
+            get :edit, params: { step: "details" }
+
+            expect(order.reload.state).to eq("address")
+          end
+        end
+      end
+
+      context "when order state is 'payment'" do
+        context "when loading address step" do
+          before do
+            order.update!(state: "payment")
+          end
+
+          it "updates the order state to address" do
+            get :edit, params: { step: "details" }
+
+            expect(order.reload.state).to eq("address")
+          end
+        end
+      end
     end
   end