gitweb: fix snapshot support

[jc: when I applied the patch I misread RFC 2616 which mildly
 recommended against using the name "gzip", which was there only
 for a historical reason.  This fixes the mistake up and uses
 the content-encoding "x-gzip" again.]

Signed-off-by: Junio C Hamano <junkio@cox.net>

gitweb/gitweb.perl

@@ -2202,7 +2202,7 @@ sub git_snapshot {
 	my $filename = basename($project) . "-$hash.tar.gz";
 
 	print $cgi->header(-type => 'application/x-tar',
-			-content-encoding => 'gzip',
+			-content-encoding => 'x-gzip',
 			'-content-disposition' => "inline; filename=\"$filename\"",
 			-status => '200 OK');