JSON:API 1.0 and 1.1 specification compliance test cases

[wopian]

There's many edge cases that kitsu currently doesn't test its spec compliance with. Discussion about a test suite cropped up at json-api/json-api#1209, but they're all Ruby solutions at the moment.

Ideally we just want generic JSON responses that contains everything the spec allows.

An initial implementation has been added in __cases__ and the Kitsu class spec for get requests.

[wopian]

Tests needed for #237