Fixed order total and shipment cost not updating after changing shipping rate

After changing shipping rate in checkouts controller, the order total and shipment cost remained the same. This commit fixes this problem.

I got no regression test as there is some problem with shipment methods in checkouts controller test. After refreshing rates [in this line](https://github.com/spree/spree/blob/master/api/spec/controllers/spree/api/checkouts_controller_spec.rb#L111), the shipment has three shipping rates, and every one's cost is 0. Any ideas for this?