Fix drag and dropping a node "forward" in the document (#1621)
Fix drag and dropping a node "forward" in the document (#1621)

When drag and dropping a node to a position earlier in the document,
the node is inserted and then removed right after.
Regenerate the key to make sure the old node is removed instead of the
new one.
ashutoshrishi/slate,ianstormtaylor/slate,isubastiCadmus/slate,ianstormtaylor/slate