Use set_charset instead of query to set the db connection charset

### Issue
Based on a forum post (http://forum.joomla.org/viewtopic.php?f=617&t=847166) there is an issue with `real_escape_string` in Joomla 2.5.
This is due to how we set up the connection charset.
On the PHP manual (http://www.php.net/manual/en/mysqli.set-charset.php) it says:
> This is the preferred way to change the charset. Using mysqli_query() to set it (such as SET NAMES utf8) is not recommended.

In J3.3 we don't have this issue because this change was already done somewhere during 3.0 alpha.

### Testing
According to the forum post, the issue can for example be seen if you use multibytes characters in a search/filter.
However it will not show if the database is set up to use utf8 charset in the connection by default (which today is standard). So it's enviroment related.