Adjust support for "application/graphql" media type so it is used only if explicitly requested #375
Description
As discussed with @andimarek and as seen in graphql/graphql-over-http#31, the "application/graphql MIME type is not yet officially registered and the spec is not ready for adoption. Most GraphQL libraries do not support this MIME type and are not considering it right now.
As a consequence, we should keep supporting it but stick to "application/json" as a default for now.