Allow access to HttpResponseHeaders

[rose-a]

fixes #90

[rose-a]

I changed it to GraphQLHttpResponse<T> : GraphQLResponse<T>, IMO this is a really clean solution.

There is an extension method AsGraphQLHttpResponse() to cast the returned GraphQLResponse<T> back to a GraphQLHttpResponse<T> (necessary if a dynamic type was used) which will throw when the cast is not possible (aka using a websocket).

[sungam3r]

Indeed, more clean.