Fix bug in Orm\Model_Nestedset::dump_tree()

Whem node had 2 and more levels of childs, next node on level 0 was child of one level up node in returned array.

Signed-off-by: Kristián Feldsam <feldsam@gmail.com>
Merge pull request #287 from feldsam/1.7/develop

Fix bug in Orm\Model_Nestedset::dump_tree()