send-email: find body-encoding correctly

In 8291db6 (git-send-email: add charset header if we add encoded 'From',
2007-11-16), "$1" is used from a regexp without using () to capture
anything in $1. Later, when that value was used, it causes a warning about
a variable being undefined, instead of using the correct value for
comparison (not that it makes difference in the current code that does not
do actual re-encoding).

Signed-off-by: Peter Valdemar Mørch <peter@morch.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -850,7 +850,7 @@ foreach my $t (@files) {
 				}
 				elsif (/^Content-type:/i) {
 					$has_content_type = 1;
-					if (/charset="?[^ "]+/) {
+					if (/charset="?([^ "]+)/) {
 						$body_encoding = $1;
 					}
 					push @xh, $_;