use context from http request

[0x616E676572]

The idea behind this is that if I use standard http mux with a middleware, for an example, to set currently authenticated user into the request's context it's not working as expected

[coveralls]

Coverage remained the same at 94.444% when pulling 577a782 on 0x616E676572:request_context into 37fc9f5 on graphql-go:master.

[chris-ramon]

lgtm 👍 , request.Context() defaults to context.Background(), so better use the request one; thanks a lot @0x616E676572