def decode_header_field v
    return unless v
    return v unless v.is_a? String
    return unless v.size < MAX_HEADER_VALUE_SIZE # avoid regex blowup on spam
    d = v.dup
    d = d.transcode($encoding, 'ASCII')
    Rfc2047.decode_to $encoding, d
  end
def decode_header_field v
    return unless v
    return v unless v.is_a? String
    return unless v.size < MAX_HEADER_VALUE_SIZE # avoid regex blowup on spam
    d = v.dup
    d = d.transcode($encoding, 'ASCII')
    Rfc2047.decode_to $encoding, d
  end
if you specify a :header, will use values from that. otherwise,
 will try and load the header from the source.