git-send-email: add charset header if we add encoded 'From'

git-send-email: add charset header if we add encoded 'From'

We sometimes pick out the original rfc822 'From' header and
include it in the body of the message. If the original
author's name needs encoding, then we should specify that in
the content-type header.

If we already had a content-type header in the mail, then we
may need to re-encode. The logic is there to detect
this case, but it doesn't actually do the re-encoding.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>