Given the header name and the value from the model, return binary
data 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.
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. If cte_type is 7bit, non-ascii binary data is CTE
encoded using the unknown-8bit charset. Otherwise the original source
header is used, with its existing line breaks and/or binary data.