git-imap-send: Strip smtp From_ header from imap message.

Cyrus imap refuses messages with a 'From ' Header.

[jc: Mike McCormack says this is fine with Courier as well.]

Signed-off-by: Markus Amsler <markus.amsler@oribi.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>