JS API export doesn’t behave as native sketch export
If you have overlapping artboards the JS export will behave differently than the native sketch export. Example code:

```
let sketch = require('sketch')
let Artboard = sketch.Artboard
let Rectangle = sketch.Rectangle
let document = sketch.getSelectedDocument()

let page = document.pages[0]
page.layers = []

let bgArtboard = new Artboard({
  frame: new Rectangle(0,0,100,100),
  parent: page,
  background: { color: "#D0021B", enabled: true, includedInExport: true }
})

let fgArtboard = new Artboard({
  frame: new Rectangle(25,25,50,50),
  parent: page,
  background: { color: "#0000FF", enabled: true, includedInExport: true }
})

const options = { formats: 'png', output: false }
const buffer = sketch.export(bgArtboard, options).toNSData()

var pasteboard = NSPasteboard.generalPasteboard();
pasteboard.clearContents();
pasteboard.setData_forType(buffer, NSPasteboardTypePNG);
```

![ExportBug](https://user-images.githubusercontent.com/4199296/67614306-adc04400-f76f-11e9-8c8e-ba10f2087db1.png)
