This is the reverse of MIME.decode_words_text_remapped() . A
single UNICODE string is provided, which is separated into
fragments and transcoded to selected character sets by this
function as needed.
Parameter encoding
Either "base64" or "quoted-printable"
(or either "b" or "q" for short).
Parameter charset
Either the name of a character set to use, or a function returning
a character set to use given a text fragment as input.