"confirmChange" prevents reordering as well as nesting

[lucioperca]

When the confirmChange property is provided and returns false for a given item and parent, AND the parent is currently expanded, the item is prohibited from being dragged below the parent altogether, rather than just preventing a parent-child relationship.

To reproduce:

1. Go to the demo (https://primetwig.github.io/react-nestable/dist/example/) and choose "Example with confirmChange".
2. Attempt to drag "Fruit box" below "Box" (without first collapsing "Box").

Expected result: "Fruit box" is allowed to be reordered below "Box". (But not marked as a child of "Box".)
Actual result: "Fruit box" is prevented from moving in any way.

Note that collapsing "Box" DOES allow me to drag "Fruit box" below it.