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.
IIRC calling `Array.prototype.slice` on `arguments` incurs performance penalty.

This is the usual way of handling optional args:

```js
if (cb === undefined && typeof options === 'function') {
  cb = options;
  options = {};
}
```