Layout cleared after save

It seems that `pptx.save()` clears customized layout.

```
pptx.setLayout({ name: 'mylayout', width: 8.5, height: 11});
// {name: "custom", width: 7772400, height: 10058400}

pptx.save();

pptx.getLayout();
// {name: "screen16x9", width: 9144000, height: 5143500}
```

Is it supposed to happen, and if there's way to preserve the user customized layout?

Thank you.