Given the header name and the value from the model, return the value
to be returned to the application program that is requesting that
header. 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.
If the value contains binary data, it is converted into a Header object
using the unknown-8bit charset. Otherwise it is returned unmodified.