fix(ws/shard): use WebSocket#close() instead of WebSocket#send() to close connection (#518)

* fix(ws/shard): use WebSocket#close() instead of WebSocket#send() to close connection

* Oops forgot the return statement

src/ws/shard.ts

@@ -228,7 +228,8 @@ async function heartbeat(
           },
         },
       );
-      return shard.ws.send(JSON.stringify({ op: 4009 }));
+
+      return shard.ws.close(4009, "Session timed out");
     }
   }