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]: 81d7171

Related: socketio/socket.io-client#1475

Author: darrachequesne

lib/socket.js

@@ -386,6 +386,7 @@ class Socket extends Emitter {
         case "ping":
           this.resetPingTimeout();
           this.sendPacket("pong");
+          this.emit("ping");
           this.emit("pong");
           break;