fixes failing pagination when using groupBy

[rufinus]

needs testing with dffierent usecases.

my use case was:

    public function buildTable(TableBuilderInterface $builder, array $options)
    {
        // [...]
        $builder->add('fields', 'text', array(
            'params' => array('GROUP_CONCAT(f.name SEPARATOR \', \')'),
            'title'  => 'Options',
            'allow_sort' => true,
            'allow_filter' => true,
        ))
        // [..]
    }

    public function getQueryBuilder(ObjectManager $entityManager, array $params)
    {
        $qb = $entityManager
            ->getRepository('Model:CustomField')
            ->createQueryBuilder('c')
            ->leftJoin('c.fields', 'f')
            ->leftJoin('c.parent', 'p')
            ->where('p is null')
            ->groupBy('c.id');

        return $qb;
    }