e-mail workflow: Message-ID is spelled with ID in both capital letters

We used to write "Message-Id:" and "Message-ID:" pretty much
interchangeably, and the header name is defined to be case
insensitive by the RFCs, but the canonical form "Message-ID:" is
used throughout the RFC documents, so let's imitate it ourselves.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
diff --git a/git-send-email.perl b/git-send-email.perl
index 5861e99..092b893 100755
--- a/git-send-email.perl
+++ b/git-send-email.perl
@@ -1530,7 +1530,7 @@
 To: $to${ccline}
 Subject: $subject
 Date: $date
-Message-Id: $message_id
+Message-ID: $message_id
 ";
 	if ($use_xmailer) {
 		$header .= "X-Mailer: git-send-email $gitversion\n";
@@ -1825,7 +1825,7 @@
 				$has_mime_version = 1;
 				push @xh, $_;
 			}
-			elsif (/^Message-Id: (.*)/i) {
+			elsif (/^Message-ID: (.*)/i) {
 				$message_id = $1;
 			}
 			elsif (/^Content-Transfer-Encoding: (.*)/i) {