Offset2D changes the orientation of a circle

```python
import cadquery as cq

part = cq.Workplane("XZ").circle(1)

show_object(part, "pre-offset", options={"color": "green"})

part = part.offset2D(1)

show_object(part, "post-offset", options={"color": "blue"})
```


It's a bit hard to see in the image, so to be clear, the pre-offset circle lies in the XZ plane, the post-offset circle lies in the XY plane when you would expect it in the XZ plane.

I can only reproduce this behaviour with circles - closed splines, two arcs making a circle and polygons all work as expected.

Using current master, 2b7f39b234df15328ad0102b1b0c15bbd4f7efb4.