http认证 返回字段 expire_at 必填

[wandun]

File: /zh_CN/access-control/authn/http.md

文档写着可选，

但是如果你不传，emqx就会出现如下问题，导致认证失败，返回，账号或者密码错误

[wandun]

版本 5.8.2

[id]

@wandun the expire_at field does present in the response, but is set to null. You need to either exclude it, or set to a valid integer value.