Fix #920 - add MQTT over Websocket client example

src/ws.h

@@ -23,3 +23,4 @@ struct mg_connection *mg_ws_connect(struct mg_mgr *, const char *url,
 void mg_ws_upgrade(struct mg_connection *, struct mg_http_message *,
                    const char *fmt, ...);
 size_t mg_ws_send(struct mg_connection *, const char *buf, size_t len, int op);
+size_t mg_ws_wrap(struct mg_connection *, size_t len, int op);