fix tagging error

weed/server/filer_server_handlers_tagging.go

@@ -81,9 +81,11 @@ func (fs *FilerServer) DeleteTaggingHandler(w http.ResponseWriter, r *http.Reque
 	// parse out tags to be deleted
 	toDelete := strings.Split(r.URL.Query().Get("tagging"), ",")
 	deletions := make(map[string]struct{})
+	if !(len(toDelete) == 1 && toDelete[0] == "") {
 		for _, deletion := range toDelete {
 			deletions[deletion] = struct{}{}
 		}
+	}
 
 	// delete all tags or specific tags
 	hasDeletion := false