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.
This has been discussed, but we'll finally be doing it for the next
major version so that we can introduce a few features that depend on
gems that don't support 1.9.
* 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
This moves away from rest-client's convention of using symbols as header
names so as to present less obfuscation as to how these are actually
named when they go over the wire.
Because headers can be injected via the bindings' API I was initially
worried that this change might break something, but upon inspection of
rest-client source, I can see now that headers take precedence as
assigned by their insertion order into the header hash, and are
"stringified" in that same loop [1]. This means that even if a user
injects a symbolized header name (`:idempotency_key`), it will still
correctly overwrite the one generated by stripe-ruby despite that using
the string format (`"Idempotency-Key"`).
[1] https://github.com/rest-client/rest-client/blob/master/lib/restclient/request.rb#L603,L625
* 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
