Added "to_unicode" to send_pgp_mime.flatten()

Added "to_unicode" to send_pgp_mime.flatten()

be-handle-mail wants unicode output, since all it's internal
processing is done with unicode.  However, the flatten calls in
send_pgp_mime work with the encoded binary string output, and
execute(sendmail, stdin=flatten(msg, to_unicode=True)) fails
with
  Exception: u
  while executing /usr/sbin/sendmail -t
  sendmail: fatal: wking(1001): No recipient addresses found in message header