Given the header name and the value from the model, return a string
containing linesep characters that implement the folding of the header
according to the policy controls. The value passed in by the email
package may contain surrogateescaped binary data if the lines were
parsed by a BytesParser. The returned value should not contain any
surrogateescaped data.
Headers are folded using the Header folding algorithm, which preserves
existing line breaks in the value, and wraps each resulting line to the
max_line_length. Non-ASCII binary data are CTE encoded using the
unknown-8bit charset.