void responseToPing(Socket socket) throws IOException {
        OutputStream out = socket.getOutputStream();
        out.write("HTTP/1.1 200 OK\n\n".getBytes());
        out.write(PING_RESPONSE.getBytes());
    }
Pings {@link HttpProxyCacheServer} to make sure it works.
VideoCache/src/main/java/com/yc/videocache/Pinger.java