Skipping before_filters present in spree_frontend 2.1.7

Looked into ways of skipping all filter, which I think is the intention.
The answer here http://stackoverflow.com/a/6063143 looked promising,
but didn't work. So adding more before_filters present in
Spree::CheckoutController (spree_frontend gem) to skip.
Adding a port to callback_url()

Following the pattern of specifying these things here.
Removing attr_accessor (previously attr_accessible) from Spree::Gateway::PxPay

attr_accessible was removed in recent versions of Rails, and this file
apparently doesn't need attr_accessor either.