header.go
func rfc2047AttributeName(s string) string {
	if !strings.Contains(s, "?=") {
		return s
	}
	pair := strings.SplitAfter(s, "?=")
	pair[0] = decodeHeader(pair[0])
	return strings.Join(pair, "")
}
func rfc2047AttributeName(s string) string {
	if !strings.Contains(s, "?=") {
		return s
	}
	pair := strings.SplitAfter(s, "?=")
	pair[0] = decodeHeader(pair[0])
	return strings.Join(pair, "")
}
// rfc2047AttributeName checks if the attribute name is encoded in RFC2047 format
// RFC2047 Example:
//     `=?UTF-8?B?bmFtZT0iw7DCn8KUwoo=?=`