Moves away from Committee and towards stripe-mock, an external
self-contained executable API stub server based on OpenAPI [1]. The
motivation here is that instead of making stripe-ruby a special
snowflake, we can use a single well-tested and feature-rich mock
implementation to drive every API's test suite.
[1] https://github.com/stripe/stripe-mock
Follows the path established in 2d75c8f by porting the rest of
stripe-ruby's tests over to OpenAPI. There are a few other changes here
where I've removed some tests that are duplicated or don't make much
sense, or reorganized how we test certain things, but this commit is
largely the same migration operation applied in bulk a few dozen test
suites.
* Add support for multiplan subscriptions:
Serialize indexed arrays into hashes with index keys in subscription create, subscription update, and upcoming invoice
Add a SubscriptionItem object that supports creation, deletion, update, listing, and retrieval
* Remove helpers that convert items array to indexed hash
* Rename the `Update` operation to `Save`
* Add the `update` class method to all saveable resources
* Add tests for update method
* Add tests for plans, invoice items, and application fees
* allow subs to be retrieved & created with new v1/subs API endpoint
* edit tests to check for url
* fix customer subscription URLs to go through v1/customers
