Subscriptions cannot be created when Zuora REST API feature turned on
Problem
When the zuora_rest_api feature is turned on, users are not able to purchase subscriptions. This feature was newly introduced by !829 but it's not currently breaking in production as the feature is not turned on at the moment.
Proposal
Adjust specs in subscriptions_controller_spec.rb to run with the feature flag turned on or off, which should highlight the bug, then fix the bug.
Result
A functional purchase process for subscriptions when the REST feature is enabled.