added example for heartbeat

examples/esp8266/WebSocketClient/WebSocketClient.ino

@@ -84,6 +84,12 @@ void setup() {
 
 	// try ever 5000 again if connection has failed
 	webSocket.setReconnectInterval(5000);
+
+  // start heartbeat (optional)
+  // ping server every 15000 ms
+  // expect pong from server within 3000 ms
+  // consider connection disconnected if pong is not received 2 times
+  webSocket.enableHeartbeat(15000, 3000, 2);
 
 }