Down arrow press on draggable Tree

[EcoFreak]

I'm submitting a bug.

[ ] bug report => Search github for a similar issue or PR before submitting
[ ] feature request => Please check if request is not on the roadmap already https://github.com/primefaces/primeng/wiki/Roadmap
[ ] support request => Please do not submit support request here, instead see http://forum.primefaces.org/viewforum.php?f=35

Current behavior
When using a tree with drag/drop, the down key does not focus the children nodes but instead focus the next sibling node.

Expected behavior
The down key should focus the children nodes if they are present. This behavior occurs when the tree does not have drag/drop feature activated.

Minimal reproduction of the problem with instructions
Using the primeng tree compoment documentation the issue is reproducable.
https://www.primefaces.org/primeng/#/tree

Down keypress on first example tree works ok (when focusing a parent node and pressing down, the first child is focused).

DragDrop key does not work ok. (when focusing a parent node and pressing down, the next sibling is focused).

I think this is an easy fix.
According to the onKeyDown function the code that handles the down arrow (40) does not check the droppableNodes property.