Return 406 on unsupported Accept header

[sungam3r]

Note: as it relates to RFC 7231, we are in compliance now -- see where it says "or disregard the header field...". Again, I like better the use of status code 406, but probably implement it in version 8, reconfiguring the middleware to perform content type negotiation prior to execution of the GraphQL request.

Originally posted by @Shane32 in #919 (comment)