I'm trying to gzip responses from GAE server, but receive null in Content-Encoding.

I have the following code:

connection = (HttpURLConnection)url.openConnection();
connection.setRequestMethod("POST");
connection.setRequestProperty("Content-Type", 
          "application/json; charset=utf-8"); //"application/json; charset=utf-8"
connection.setRequestProperty("Accept-Encoding", "gzip");
connection.setRequestProperty("User-Agent", "gzip");
connection.setUseCaches (false);
connection.setDoInput(true);
connection.setDoOutput(true);

//write
//read

System.out.println("Content-Encoding " + connection.getContentEncoding());

I've read that on GAE servers do compressing automatically. So what can be the problem?

java google-app-engine servlets
Edit
Report