[api] Convert payment_method_id to string in CheckoutsController

Without this, payment information coming in through JSON will result in a payment which has no source
[api] Do not wrap parameters in OrdersController

This fixes the problem where POST /api/orders with no parameters would result in the parameters coming to the controller as params[:order], courtesy of this:

https://github.com/rails/rails/blob/master/actionpack/lib/action_controller/metal/params_wrapper.rb

Disabling it means that we can't submit JSON parameters without a root element, but really we don't advise people to do that, so I think it's a small thing to lose
Fix typo in Spree::ReturnAuthorization model
Destroy asocaited adjustments when order is changed to move out of taxable zones.

Fixes #4840
Allow token-based payment sources in Payment::Processing

Related to #4598 and ZD#4573