Fix admin payment form not being displayed on payment method change

When there are multiple payment methods, clicking the radio button for a payment method other than the first causes the payment forms to not be displayed.

How to replicate in sandbox:
1) Create an order through the admin until the payment state is reached
2) Create a new payment for the order (at this point the credit card form should be visible)
3) Click the Check radio button
4) Click the Credit Card radio button
5) The credit card form is no longer visible
exchange expires_at to starts_at to fix test
Move set_shipments_cost call to Order#update_from_params

This stops the method from being called whenever Order#checkout_steps is called and thereby reduces some queries

This needs to be called in the checkout_specs to ensure that the state isn't jumped as long as this method is called.

Fixes #4898
Fixes #4979
Shipment cost is now included in spec totals.
Merge pull request #4977 from alexandremcosta/fix_promotion_spec

exchange expires_at to starts_at to fix test
Merge pull request #4974 from bonobos/fix/admin-payment-method-switch

Fix admin payment form not being displayed on payment method change