Add Apple Pay domain management support.

[ethier]

Summary

• Add ApplePayGateway, with registered_domains(), register_domain() and unregister_domain() method support.
• Update HTTP to mark 204 as a success HTTP code
• Add tests for mock requests

Checklist

• [x] Added changelog entry
• [x] Ran unit tests (nosetests tests/unit)

[ethier]

Thanks @crookedneighbor! Makes sense to wait until after the holidays. Appreciate the help. Have a good new year!

[crookedneighbor]

This is out in 3.58.0. Thanks for your work on this!

[ethier]

Thanks @crookedneighbor! Appreciate the help on this.