Fix fxpay API views, fix CORS (bug 1104371)

The missing serializer_class causes a 500 and
I'm not sure how long that has been broken.

http://sentry.dmz.phx1.mozilla.com/marketplace-payments-alt/payments-altallizomorg/group/21620/
Merge pull request #2754 from kumar303/api-fix

Fix fxpay API views, fix CORS (bug 1104371)