fix(example): set content-type in the userinfo response

This change sets the `content-type` header to `application/json` for the response sent to the browser in the app example.
This enables pretty-printing of the userinfo json document in at least Chromium.

example/client/app/app.go

@@ -108,6 +108,7 @@ func main() {
 			http.Error(w, err.Error(), http.StatusInternalServerError)
 			return
 		}
+		w.Header().Set("content-type", "application/json")
 		w.Write(data)
 	}