Clear shipping_method_id from order when restarting checkout

If the order is allowed to retain a shipping_method_id, then subsequent
saves of the order will cause a new shipment to be initialised. Seems to
only happen for delivery shipping methods. This is undesirable because
fees for the new shipment will appear in the checkout summary, which is
not smart enough to recognise existing shipment fees and adjust the order
total accordingly.

app/controllers/checkout_controller.rb

@@ -163,6 +163,7 @@ class CheckoutController < Spree::CheckoutController
   def restart_checkout
     return if @order.state == 'cart'
     @order.restart_checkout! # resets state to 'cart'
+    @order.update_attributes!(shipping_method_id: nil)
     @order.shipments.with_state(:pending).destroy_all
     @order.payments.with_state(:checkout).destroy_all
     @order.reload

spec/controllers/checkout_controller_spec.rb

@@ -223,6 +223,7 @@ describe CheckoutController do
     let!(:payment_failed) { create(:payment, order: order, state: 'failed') }
 
     before do
+      order.update_attribute(:shipping_method_id, shipment_pending.shipping_method_id)
       controller.instance_variable_set(:@order, order.reload)
     end
 
@@ -242,12 +243,14 @@ describe CheckoutController do
       # 'pending' when the order has not been completed, so this is not a case that requires testing.
       it "resets the order state, and clears incomplete shipments and payments" do
         expect(order).to receive(:restart_checkout!).and_call_original
+        expect(order.shipping_method_id).to_not be nil
         expect(order.shipments.count).to be 1
         expect(order.adjustments.shipping.count).to be 1
         expect(order.payments.count).to be 2
         expect(order.adjustments.payment_fee.count).to be 2
         controller.send(:restart_checkout)
         expect(order.reload.state).to eq 'cart'
+        expect(order.shipping_method_id).to be nil
         expect(order.shipments.count).to be 0
         expect(order.adjustments.shipping.count).to be 0
         expect(order.payments.count).to be 1