private String getPingUrl() {
        return String.format(Locale.US, "http://%s:%d/%s", host, port, PING_REQUEST);
    }
Pings {@link HttpProxyCacheServer} to make sure it works.

@author Alexey Danilov (danikula@gmail.com).
dkplayer-videocache/src/main/java/com/danikula/videocache/Pinger.java