simplify the gzip optimization logic
simplify the gzip optimization logic

fix https://github.com/chrislusf/seaweedfs/issues/436

hxiaodon/seaweedfs,k85169336/seaweedfs,hxiaodon/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs,k85169336/seaweedfs