git-send-email: do not pass custom Date: header

We already generate a Date: header based on when the patch was
emailed.  git-format-patch includes the Date: header of the
patch.  Having two Date: headers is just confusing, so we
just use the current Date:

Often the mailed patches in a patch series are created over a
series of several hours or days, so the Date: header from the
original commit is incorrect for email, and often far off enough
for spam filters to complain.
Signed-off-by: NEric Wong <normalperson@yhbt.net>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-send-email.perl

@@ -514,7 +514,7 @@ sub send_message
 						$2, $_) unless $quiet;
 					push @cc, $2;
 				}
-				elsif (/^[-A-Za-z]+:\s+\S/) {
+				elsif (!/^Date:\s/ && /^[-A-Za-z]+:\s+\S/) {
 					push @xh, $_;
 				}