As of graphql/graphql-over-http#215, the official media type for GraphQL HTTP responses is now `"application/graphql-response+json"` instead of `"application/graphql+json"`. The latter is now deprecated and support will be removed in the future. This commit now favors the new media type. HTTP Clients are still supposed to send requests with the `"application/json"` content type. Closes gh-563