Adding border or padding to the sortable-list adds extra height while dragging

### Description

Using the demo code, if we add border or padding to `<sortable-list>`, its height grows while dragging an item. Of course there is workarounds or probably that's not the intended usage, but IMHO that's not a expected behavior.

### Live Demo
http://jsbin.com/towofic/4/edit?html,output

In the demo, notice how the green box below is pushed down while dragging an item.

