Making options argument optional per docs

If options are not passed to the access-token.findForRequest() function, then the callback argument will be undefined.  This makes the options truly optional as the docs prescribe.
> Any idea of the benefit of the "loopback way"?

I am not sure as I don't use it. I suppose it supports arbitrary number of optional arguments before the last callback arg. I.e. the following call will work correctly too:

```js
AccessToken.findForRequest(req, options, 1, 2, 3, callbackFn);
```