// returned without modifications.
func CanonicalMIMEHeaderKey(s string) string {
	// Quick check for canonical encoding.
	upper := true
	for i := 0; i < len(s); i++ {
		c := s[i]
		if !validHeaderFieldByte(c) {
			return s
		}
		if upper && 'a' <= c && c <= 'z' {
			s, _ = canonicalMIMEHeaderKey([]byte(s))
			return s
		}
		if !upper && 'A' <= c && c <= 'Z' {

	// users to provide RFC 3339 compliant dates.
	retDate, err = amztime.ISO8601Parse(dateStr)
	if err != nil {
		return rmode, r, ErrInvalidRetentionDate
	}
	_, replReq := h[textproto.CanonicalMIMEHeaderKey(xhttp.MinIOSourceReplicationRequest)]

	t, err := UTCNowNTP()
	if err != nil {
		lockLogIf(context.Background(), err)
		return rmode, r, ErrPastObjectLockRetainDate
	}