Fix infinite loop in spec

The spec was setting the order's state to "complete" but didn't save
that state to the database. The new locking mechanism is was reloading
the order which loaded the cart state again. And since the order.next
method was mocked to just return true, the controller was trying to do
that in an infinite loop.

spec/controllers/checkout_controller_spec.rb

@@ -130,6 +130,7 @@ describe CheckoutController, type: :controller do
     context 'when completing the order' do
       before do
         order.state = 'complete'
+        order.save!
         allow(order).to receive(:update_attributes).and_return(true)
         allow(order).to receive(:next).and_return(true)
         allow(order).to receive(:set_distributor!).and_return(true)