Allow new enum values without blowing up

[richmolj]

If an API adds an enum, and that enum appears in the response, this client will throw an unexpected enum value until the schema is updated. Clients who don't control the backend will blow up when the API provider makes a backwards-compatible change.

It's caused by this line.

I'd be happy to submit a PR removing this check, but wanted to check beforehand if this should be a configuration option?