Manager: override logger settings only when using the built in Logger

We are using log4js-node for logging in our application.

```js
var log = log4js.getLogger('socket');
log.setLevel('ERROR');

io.set('logger', log);
```

Now, when socket.io gets the logger, it overrides the logger settings. This breaks the log4js-node logger instance and results in an exception. This PR will prevent this and just return the logger which was set through io.set()