Bug fixed grid-0018: the head should align correct in ie8
B70-ZK-2128: Frozen Tree - cells disappearing after replacing the columns twice http://tracker.zkoss.org/browse/ZK-2128
merge branch of 'B70-ZK-2128' of https://github.com/danchen0720/zk.git
Merge remote-tracking branch 'RaymondChao/B70-ZK-2128'