Fix crash on hartbeat on closing connection

Fix heartbeat incorrectly asserting it cannot be sent on a connection in
the closing state.

lib/link/connectors.js

@@ -43,7 +43,7 @@ class WebSocketBaseConnector extends events.EventEmitter {
 	}
 
 	_doHeartbeat() {
-		this._check(["connected"]);
+		this._check(["connected", "closing"]);
 		if (Date.now() - this._lastHeartbeat > 2000 * this._heartbeatInterval) {
 			console.log("SOCKET | closing after heartbeat timed out");
 			this._socket.close(1008, "Heartbeat timeout");