'The same as fold if cte_type is 7bit, except that the returned value is
bytes.
If cte_type is 8bit, non-ASCII binary data is converted back into
bytes.  Headers with binary data are not refolded, regardless of the
refold_header setting, since there is no way to know whether the binary
data consists of single byte characters or multibyte characters.'

def fold_binary(self, name, value):

    folded = self._fold(name, value, refold_binary=(self.cte_type == '7bit'))
    return folded.encode('ascii', 'surrogateescape')
