weed filer: http HEAD response header add ETag

fix https://github.com/chrislusf/seaweedfs/issues/892

weed/server/filer_server_handlers_read.go

@@ -52,6 +52,7 @@ func (fs *FilerServer) GetOrHeadHandler(w http.ResponseWriter, r *http.Request,
 	if r.Method == "HEAD" {
 		w.Header().Set("Content-Length", strconv.FormatInt(int64(filer2.TotalSize(entry.Chunks)), 10))
 		w.Header().Set("Last-Modified", entry.Attr.Mtime.Format(http.TimeFormat))
+		setEtag(w, filer2.ETag(entry.Chunks))
 		return
 	}