Fix UTF-7 XSS vulnerability.

If you don't specify a charset in your application page, then it is susceptible to inherit the charset in the parent page via iframes. So, if you accidently land on an evil site, an attacker might be able to steal your application session since your usual XSS prevention stuff [<,>,",',etc] will not filter the utf-7 encoded chars and XSS will execute in your vulnerable domain.