Use generated API responses in stripe-ruby tests

This patch introduces a narrow working slice of tests that use data
generated from within the API itself to produce sample responses from a
stubbed out API in the test suite. This has a few advantages over the
current situation:

* More accurate test data back from the stub. The current `TestData`
  system is extremely prone to eroding (this is the biggest advantage).
* Basic validation gets run on incoming request parameters.
* Stubbing is run at a lower level (the HTTP library instead of the
  `Stripe` module) so we get a little more certainty that things work as
  expected.
* Contributing new tests (especially for new resources) becomes very
  easy because no sections of `TestData` are required.

Replacing the entire suite will take a few hours and involving a lot of
copy and pasting, but is entirably doable and after we're done we can
remove the entire `TestData` module and the monkey patching on the
`Stripe` module.

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