fix: return cachego.ErrCacheExpired

[rfyiamcool]

what

I think return cachego.ErrCacheExpired when kv is expired，The business layer only needs to judge that the returned error is cachego.ErrCacheExpired.

res, err := cache.Fetch(key)
if err == cachego.ErrCacheExpired {
        value := dao.Select(key)
        cache.Save(key, value, 10 * time.Hour)
}
...

[faabiosr]

Thanks for the contribution!