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.
Header folding is controlled by the refold_source policy setting. A
value is considered to be a 'source value' if and only if it does not
have a 'name' attribute (having a 'name' attribute means it is a header
object of some sort). If a source value needs to be refolded according
to the policy, it is converted into a custom header object by passing
the name and the value with any linesep characters removed to the
header_factory method. Folding of a custom header object is done by
calling its fold method with the current policy.
Source values are split into lines using splitlines. If the value is
not to be refolded, the lines are rejoined using the linesep from the
policy and returned. The exception is lines containing non-ascii
binary data. In that case the value is refolded regardless of the
refold_source setting, which causes the binary data to be CTE encoded
using the unknown-8bit charset.