Fix set header to JSON earlier in GraphQL response

Update the GraphQL handler to set the Response Header to JSON ealier for
error messages to be returned as JSON and not text/html.

Fixes #519

handler/graphql.go

@@ -299,6 +299,7 @@ func (gh *graphqlHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	w.Header().Set("Content-Type", "application/json")
 	var reqParams params
 	switch r.Method {
 	case http.MethodGet:
@@ -320,7 +321,6 @@ func (gh *graphqlHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusMethodNotAllowed)
 		return
 	}
-	w.Header().Set("Content-Type", "application/json")
 
 	ctx := r.Context()