Move url_helpers inside affirm_payload_json
Move url_helpers inside affirm_payload_json

The inclusion of `Spree::Core::Engine.routes.url_helpers` in `AffirmHelper` raises
an error when `rails/mailer` is visited.
Moving it inside the method resolves the problem.

rel https://github.com/solidusio-contrib/solidus_affirm/issues/10

StemboltHQ/solidus_affirm,StemboltHQ/solidus_affirm,StemboltHQ/solidus_affirm