I'm wondering if there is any way to find out what the actual reply from volumio could be.
The reply from the server should be the same regardless of what made the request, so use your favorite browser, which is smart enough to know that it needs to make a GET request.
A server just sits there, parsing data that arrives on the port it is listening to, and passing it to the proper script/application, returning whatever output the script/application generates to the requesting client.
If you make a request that the server doesn't understand (like "Hello from oblivious"), it is free to not respond at all. If you make a request it can't handle (like "GET me/some/ice/cream HTTP/4.6"), it will generate a reply.