feature: Implement GraphQL response type

[calvincestari]

This introduces a new media type that would be used for GraphQL responses, see the draft GraphQL over HTTP spec.

It looks like the work should generally be simply:

1. Modify the accept header to include the new media type signalling to the server that the client can accept responses of that type.
2. Add application/graphql-response+json to any content-type validation we do before parsing a server response.
3. Errors - TBD

[calvincestari]

@martinbonnin - I think you said something earlier about taking action on errors when the response is of the new type, is that correct?

[martinbonnin]

@calvincestari correct, depends what your behaviour is right now but typically you should throw an error if HTTP code != 200. With the advent of graphql-response+json, these responses can be parsed and exposed to the user instead of throwing.