git-send-email: show all headers when sending mail

As a git newbie, it was confusing to set an In-Reply-To header but then
not see it printed when the git-send-email command was run.

This patch prints all headers that would be sent to sendmail or an SMTP
server instead of only printing From, Subject, Cc, To.  It also removes
the now-extraneous Date header after the "Log says" line.

Added test to t/t9001-send-email.sh.

Signed-off-by: David D. Kilzer <ddkilzer@kilzer.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-send-email.perl b/git-send-email.perl
index fd0a4ad..47afc56 100755
--- a/git-send-email.perl
+++ b/git-send-email.perl
@@ -643,7 +643,7 @@
 	if ($quiet) {
 		printf (($dry_run ? "Dry-" : "")."Sent %s\n", $subject);
 	} else {
-		print (($dry_run ? "Dry-" : "")."OK. Log says:\nDate: $date\n");
+		print (($dry_run ? "Dry-" : "")."OK. Log says:\n");
 		if ($smtp_server !~ m#^/#) {
 			print "Server: $smtp_server\n";
 			print "MAIL FROM:<$raw_from>\n";
@@ -651,7 +651,7 @@
 		} else {
 			print "Sendmail: $smtp_server ".join(' ',@sendmail_parameters)."\n";
 		}
-		print "From: $sanitized_sender\nSubject: $subject\nCc: $cc\nTo: $to\n\n";
+		print $header, "\n";
 		if ($smtp) {
 			print "Result: ", $smtp->code, ' ',
 				($smtp->message =~ /\n([^\n]+\n)$/s), "\n";