fix issue about expandAll

Issue Info:

when you load the special data,  if you use the  expandAll() method,
some node's DOM will be appended twice.

example data:  (expand.html)

var zNodes =[
{ id:1, pId:0, name:"ordinary parent 1", t:"I am ordinary, just expand
or collapse on me", open:false},
{ id:11, pId:1, name:"leaf node 1-1", t:"..."},
{ id:12, pId:1, name:"ordinary parent 2", t:"I am ordinary, just expand
or collapse on me", open:false},
{ id:121, pId:12, name:"parent node - 121", t:"...", open:true},
{ id:1211, pId:121, name:"parent node - 1211", t:"...", open:false},
{ id:1212, pId:1211, name:"leaf node - 1212", t:"..."},
{ id:1213, pId:121, name:"parent node - 1213", t:"...", open:false},
{ id:1214, pId:1213, name:"leaf node - 1214", t:"..."},
{ id:123, pId:12, name:"leaf node 2-3", t:"..."},
{ id:13, pId:1, name:"leaf node  1-3", t:"..."},
{ id:2, pId:0, name:"can't collapse me", t:"can't collapse me...Unless
you use expandAll method.", open:true, collapse:false},
{ id:21, pId:2, name:"leaf node 2-1", t:"You can't collapse my parent"},
{ id:22, pId:2, name:"leaf node 2-2", t:"You can't collapse my parent"},
{ id:23, pId:2, name:"leaf node 2-3", t:"You can't collapse my parent"},
{ id:3, pId:0, name:"can't expand me", t:"can't expand me...Unless you
use expandAll method.", open:false, expand:false},
{ id:31, pId:3, name:"leaf node 3-1", t:"OMG, you can see me!! Do you
use the expandAll method?"},
{ id:32, pId:3, name:"leaf node 3-2", t:"OMG, you can see me!! Do you
use the expandAll method?"},
{ id:33, pId:3, name:"leaf node 3-3", t:"OMG, you can see me!! Do you
use the expandAll method?"},
{ id:4, pId:0, name:"empty parent 1", t:"I have nothing...",
isParent:true, open:false}
];