Change default response content type to match current draft spec

[Shane32]

In graphql/graphql-over-http#215 the default response media type was changed from application/graphql+json to application/graphql-response+json.

• Change media type to 'application/graphql-response+json' graphql/graphql-over-http#215

Below is the new spec (merged 1 day after GraphQL.NET Server v7 was released):

Media Types

The following are the officially recognized GraphQL media types to designate using the JSON encoding for GraphQL responses:

Name	Description
application/graphql-response+json	The preferred type for server responses; better HTTP compatibility
application/json	An alternative type for responses (to support legacy clients)

We should release 7.1 promptly to reduce the impact of this change.