* Add 'charset' to Content-Type headers on various HTTP error responses
to forestall additional UTF-7-autodetect XSS issues. Probably not an
issue on Apache 2.0+, but most servers send only 'text/html' by default
when the script didn't specify more details.
This fixes an issue with the Ajax interface error message on MSIE when
$wgUseAjax is enabled (not default configuration); this UTF-7 variant
on a previously fixed attack vector was discovered by Moshe BA from BugSec:
http://www.bugsec.com/articles.php?Security=24
* Trackback responses now specify XML content type