memory leak on reconnection when `closeOnBeforeunload` is true

[rluvaton]

Describe the bug
It looks like there is a memory leak of closeOnBeforeunload listener in case it fails to connect to WebSocket and tries to connect again

Sorry for not sticking with the issue template but I'm going to fix it if you think that it is indeed a memory leak

---

So why the memory leak? I've noticed while debugging my application that the beforeunload listener keeps adding up by socket io client

As we see here:

engine.io-client/lib/socket.ts

Lines 355 to 366 in dfee8de

	addEventListener(
	"beforeunload",
	() => {
	if (this.transport) {
	// silently close the transport
	this.transport.removeAllListeners();
	this.transport.close();
	}
	},
	false
	);
	}

But, in the onClose we don't remove that listener:

engine.io-client/lib/socket.ts

Lines 894 to 932 in dfee8de

	private onClose(reason: string, description?: CloseDetails | Error) {
	if (
	"opening" === this.readyState ||
	"open" === this.readyState ||
	"closing" === this.readyState
	) {
	debug('socket close with reason: "%s"', reason);
	// clear timers
	this.clearTimeoutFn(this.pingTimeoutTimer);
	// stop event from firing again for transport
	this.transport.removeAllListeners("close");
	// ensure transport won't stay open
	this.transport.close();
	// ignore further transport communication
	this.transport.removeAllListeners();
	if (typeof removeEventListener === "function") {
	removeEventListener("offline", this.offlineEventListener, false);
	}
	// set ready state
	this.readyState = "closed";
	// clear session id
	this.id = null;
	// emit close event
	this.emitReserved("close", reason, description);
	// clean buffers after, so users can still
	// grab the buffers on `close` event
	this.writeBuffer = [];
	this.prevBufferLen = 0;
	}
	}

just making sure that I'm correct and if so I'll create a PR to fix that...

[darrachequesne]

Good catch, thanks! This should be fixed by 99925a4, included in version 6.2.3.