git-send-email: treat field names as case-insensitively

Field names like To:, Cc:, etc. are case-insensitive; use a
case-insensitive regexp to match them as such.

Previously, git-send-email would fail to pick-up the addresses when
in-body "fake" headers with different cases (e.g. lowercase "cc:")
are manually inserted to the messages it was asked to send, even
though the text will still show them.

Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -1227,25 +1227,25 @@ sub send_message {
 		}
 
 		if (defined $input_format && $input_format eq 'mbox') {
-			if (/^Subject:\s+(.*)$/) {
+			if (/^Subject:\s+(.*)$/i) {
 				$subject = $1;
 			}
-			elsif (/^From:\s+(.*)$/) {
+			elsif (/^From:\s+(.*)$/i) {
 				($author, $author_encoding) = unquote_rfc2047($1);
 				next if $suppress_cc{'author'};
 				next if $suppress_cc{'self'} and $author eq $sender;
 				printf("(mbox) Adding cc: %s from line '%s'\n",
 					$1, $_) unless $quiet;
 				push @cc, $1;
 			}
-			elsif (/^To:\s+(.*)$/) {
+			elsif (/^To:\s+(.*)$/i) {
 				foreach my $addr (parse_address_line($1)) {
 					printf("(mbox) Adding to: %s from line '%s'\n",
 						$addr, $_) unless $quiet;
 					push @to, sanitize_address($addr);
 				}
 			}
-			elsif (/^Cc:\s+(.*)$/) {
+			elsif (/^Cc:\s+(.*)$/i) {
 				foreach my $addr (parse_address_line($1)) {
 					if (unquote_rfc2047($addr) eq $sender) {
 						next if ($suppress_cc{'self'});
@@ -1267,7 +1267,7 @@ sub send_message {
 			elsif (/^Message-Id: (.*)/i) {
 				$message_id = $1;
 			}
-			elsif (!/^Date:\s/ && /^[-A-Za-z]+:\s+\S/) {
+			elsif (!/^Date:\s/i && /^[-A-Za-z]+:\s+\S/) {
 				push @xh, $_;
 			}