dashboard流控规则集群模式下保存不生效

环境：dashboard版本1.8.5

1、流控规则->新建流控规则，选择集群模式，保存
2、修改这个流控规则，去掉集群模式，保存。
3、再编辑这个流控规则，仍然是集群模式，也就是第2步的集群模式保存不生效

Start at the dashboard flow-control rule create and edit entry points, then reproduce the reported cluster-mode sequence in Sentinel dashboard 1.8.5. Trace why clearing cluster mode is not retained after saving; done means reopening the rule shows cluster mode disabled after the change is saved.

Written by the indexing model from the issue text.