getHttpResponseCode() returns -1 in android 2.2

In my android app, I am trying to pull data from server by doing a POST request.

I'm using HttpURLConnection class to make the requests as Apache's HttpClient is no longer maintained by android.

Here's what I'm doing.

private boolean callWS() {
    try {

        // To avoid the bug in httpurlconnection prior froyo which
        // causes the getInputStream to return headers along with response
        if (Build.VERSION.SDK_INT < 8)
            System.setProperty("http.keepAlive", "false");

        mHttpResponseCode = 0;
        mErrorMessage = "";

        // Initialize connection
        URL connectURL = new URL(mServerUrl);

        HttpURLConnection conn = (HttpURLConnection) connectURL.openConnection();
        conn.setDoInput(true);
        conn.setDoOutput(true);
        conn.setUseCaches(false);
        conn.setInstanceFollowRedirects(true);
        conn.setReadTimeout(30000); 
        conn.setConnectTimeout(15000); 
        conn.setRequestMethod("POST");

        // Set some headers
        connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
        connection.setRequestProperty("Accept-Encoding", "deflate, gzip");
        connection.setRequestProperty("Content-Length", mParameters.length() + "");

        // Connect to host
        conn.connect();

        // Write parameters to connection
        OutputStreamWriter writer = new OutputStreamWriter(conn.getOutputStream());
        writer.write(mParameters);
        writer.flush();
        writer.close();

        // Wait for http response code
        mHttpResponseCode = conn.getResponseCode();

        // Read response from connection
        BufferedInputStream bis = new BufferedInputStream(conn.getInputStream());
        ByteArrayBuffer baf = new ByteArrayBuffer(50);
        int read = 0;
        int bufSize = 1024;
        byte[] buffer = new byte[bufSize];

        while (true) {
            read = bis.read(buffer);
            if (read == -1)
                break;
            baf.append(buffer, 0, read);
        }

        // Decompress gzipped response
        if (conn.getHeaderField("Content-Encoding") != null && conn.getHeaderField("Content-Encoding").contains("gzip"))
            mResponseString = decompress(baf.toByteArray());
        else
            mResponseString = new String(baf.toByteArray());

        mResponse.setResponse(mResponseString);         
        isWSCallSuccessfull = true;
    } catch(UnknownHostException unknownHostException) {
        isWSCallSuccessfull = false;
        mErrorMessage = "Unknown host exception";
        unknownHostException.printStackTrace();
        mLogger.putStacktrace(unknownHostException);
    } catch(SocketException socketException) {
        isWSCallSuccessfull = false;
        mErrorMessage = "Socket Exception";
        socketException.printStackTrace();
        mLogger.putStacktrace(socketException);
    } catch(SocketTimeoutException socketTimeOutException) {
        isWSCallSuccessfull = false;
        mErrorMessage = "Socket Timeout Exception";
        socketTimeOutException.printStackTrace();
        mLogger.putStacktrace(socketTimeOutException);
    } catch(SSLException sslException) {
        isWSCallSuccessfull = false;
        mErrorMessage = "SSL Exception";
        sslException.printStackTrace();
        mLogger.putStacktrace(sslException);
    } catch(IOException ioException) {
        isWSCallSuccessfull = false;
        mErrorMessage = "IO Exception " + ioException.getMessage();
        ioException.printStackTrace();
        mLogger.putStacktrace(ioException);
    }

    mResponse.setHttpResponseCode(mHttpResponseCode);
    mResponse.setErrorMessage(mErrorMessage);
    mResponse.isWSCallSuccessful(isWSCallSuccessfull);

    return isWSCallSuccessfull;
}

This works fine on every device except devices running 2.2 (did not try it on 2.1).

In 2.2, it works fine. But if I leave this part of code idle for more than 30s, it returns me with -1 as http response code the very next time.

Another thing to note is that this happens only with HTTPS urls and not with HTTP Urls. I do not want to use HttpsURLConnection class because at times I may want to use http as well.

I'm not closing the connection just to keep the connection alive. What am I doing wrong?