func (e WordEncoder) bEncode(buf *strings.Builder, charset, s string) {
	w := base64.NewEncoder(base64.StdEncoding, buf)
	// If the charset is not UTF-8 or if the content is short, do not bother
	// splitting the encoded-word.
	if !isUTF8(charset) || base64.StdEncoding.EncodedLen(len(s)) <= maxContentLen {
		io.WriteString(w, s)
		w.Close()
		return
	}

	var currentLen, last, runeLen int
	for i := 0; i < len(s); i += runeLen {