[Pager] Reset orderBy in order to avoid 'Mixing of GROUP columns'

In some situation having an orderBy in the query raise the "Mixing of GROUP column" Doctrine exception in trying to get results count.

Eg:
```
[2013-04-22 11:11:56] request.CRITICAL: Doctrine\DBAL\DBALException: An exception occurred while executing 'SELECT count(DISTINCT r0_.id) AS sclr0 FROM race_entry r0_ LEFT JOIN race r1_
ON r0_.race_id = r1_.id LEFT JOIN user u2_ ON r0_.createdBy_id = u2_.id ORDER BY r0_.totalPoints DESC, r0_.totalSeconds ASC':
SQLSTATE[42000]: Syntax error or access violation: 1140 Mixing of GROUP columns (MIN(),MAX(),COUNT(),...) with no GROUP columns is illegal if there is no GROUP BY clause (uncaught except
ion) at /usr/local/webserver/php_sites.com/test/deploy/releases/20130422090406/vendor/doctrine/dbal/lib/Doctrine/DBAL/DBALException.php line 47 [] []
```

Is it possible to merge also to branch 2.1?