AccessLog can not flush into file until I stop the app

The configure the accesslog as follow：
`func MakeAccessLog(path string) *accesslog.AccessLog {
pathToAccessLog := "./access_log.%Y%m%d"
w, err := rotatelogs.New(
pathToAccessLog,
rotatelogs.WithMaxAge(24*time.Hour),
rotatelogs.WithRotationTime(time.Hour))
if err != nil {
panic(err)
}
ac := accesslog.New(bufio.NewWriter(w))
ac.AddOutput(os.Stdout)
ac.Delim = '-'
ac.TimeFormat = "2006-01-02 15:04:05"
ac.Async = true
ac.IP = true
ac.BytesReceivedBody = true
ac.BytesSentBody = true
ac.BytesSent = false
ac.BytesReceived = false
ac.BodyMinify = true
ac.RequestBody = false
ac.ResponseBody = false
ac.KeepMultiLineError = true
ac.PanicLog = accesslog.LogHandler
ac.SetFormatter(&accesslog.JSON{
HumanTime: true,
})
return ac
}
//use it in my app
ac := common.MakeAccessLog("./access.log")
defer ac.Close()
app.UseRouter(ac.Handler)`
when I access the app, the standout ouput the accesslog normally,but the log file didn't create at the app start,but when I stopped the app ,the file and the log reocord appeared.

Reproduce the delayed file output using the shown accesslog.New, bufio.NewWriter, and rotatelogs.New setup, then trace how app.UseRouter and ac.Close affect writes. Compare the stdout and file outputs while the app is running; done means access records appear in the rotated file before shutdown.

Written by the indexing model from the issue text.