adjustToFit method not work for Artboard
It looks like the methods inherits from `Group`, but it just not works.
I find it works by following script
```javascipt
document.sketchObject.actionsController().actionForID("MSResizeArtboardToFitAction").resizeArtboardToFit(nil)
```
but should ensure the `artboard` layer is selected..

Did I miss something or is this a bug? Thanks!