Encode a single header line ONLY. Do NOT include the \r\n.
Returns a string of one or more lines including the trailing \r\n.
Field-name will not be encoded, must be less than 62 chars.
The fieldBody is treated as "unstructured text",
which is suitable only for the field names "Subject" and "Comments".
We do NOT tokenize into structured fields.
To make things easy, we either encode the whole field body as RFC 2047,
or don't encode at all. If it's too long for a single line, we
encode it, even if we didn't otherwise have to.
We don't do quoted-string.
This will not split multibyte chars, including supplementary chars,
TODO this will not work for quoting structured text
such as recipient names on the "To" and "Cc" lines.