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.
* 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
