TreeModel selection size is incorrect when tree node is not rendered yet

Reproducible sample on zkfiddle: http://zkfiddle.org/sample/353rb3u/3-ZK-Tree-selection-problem

Debug Information:
In Tree.java line 2541 will clear all selection when curSeldItems size equals 1, which is not correct. It should consider if all the children nodes of treeitem are not in selected status.