type credentialHeader struct {
	accessKey string
	scope     struct {
		date    time.Time
		region  string
		service string
		request string
	}
}

// Return scope string.
func (c credentialHeader) getScope() string {
	return strings.Join([]string{
		c.scope.date.Format(yyyymmdd),
		c.scope.region,
		c.scope.service,
		c.scope.request,
	}, SlashSeparator)
}