fix: emit ping when receiving a ping from the server
fix: emit ping when receiving a ping from the server

The "ping" event was forgotten when reversing the ping-pong mechanism
in [1]. It will now be properly emitted when receiving a ping from the
server.

[1]: https://github.com/socketio/engine.io-client/commit/81d7171c6bb4053c802e3cc4b29a0e42dcf9c065

Related: https://github.com/socketio/socket.io-client/issues/1475

socketio/engine.io-client,socketio/engine.io-client,Automattic/engine.io-client,socketio/engine.io-client