Spree 1-3-stable compatable process_payments! and payment method

Corrects process_payments! ignoring order total when processing entered credit cards. This was causing multiple charges to go through on occasion.

Also adds the payment method to the order decorator since that method was removed from the Spree Order model in commit:
https://github.com/spree/spree/blob/3d0fa4ff434569ff8491aba690fde87e577ec19c/core/app/models/spree/order.rb

This was causing a method undefined error when spree_store_credits was invoked.