Add charset= support in the Mysql PDO DSN

Fixes Issue #1309

Package built for centos 6.5 http://www.hop5.in/yum/el6/hhvm-2.3.2-3.el6.x86_64.rpm with this patch

```
[bash $] hhvm  hphp/test/zend/bad/ext/pdo_mysql/tests/pdo_mysql_connect_charset.php
done!
[bash ]$ cat $_.expectf
done!
```