public function detach break the nested set tree [patch attached]

If I detach a node from the tree using $node->detach(); it set the left and right value to zero without shifting, creating a possible tree corruption.
It still work somehow because inserting a new node will shift the value but keep the hole in the tree.

I've looked to the original commit in Trac (http://trac.doctrine-project.org/changeset/4089) and I don't think the original idea of the commit was to leave a hole.
I've added 3 lines of code (that I copied from the delete function which works properly), the coresponding patch is joigned to the bug ticket