Modification to browse virtual_fields on PgRow converter when data come from postgresql

In this case below :

    class OrderMap extends BaseOrderMap
    {
        public function findWithDetails($orderId)
        {
            $orderId = (int) $orderId;
            
            $this->addVirtualField('details', 'order_detail[]');
            
            $mapOrderDetail = $this->connection->getMapFor('OrderDetail');
            $mapOrderDetail->addVirtualField('product', 'product');
            
            $this->connection->getDatabase()->registerConverter(
                        'OrderDetail',
                        new \Pomm\Converter\PgEntity($mapOrderDetail),
                        array('order_detail')
                    );
            
            $mapProduct = $this->connection->getMapFor('Product');
            
            $this->connection->getDatabase()->registerConverter(
                        'Product',
                        new \Pomm\Converter\PgEntity($mapProduct),
                        array('product')
                    );
            
            $sql = 'WITH details AS (
                        SELECT od.*, p AS product
                        FROM :order_detail_table AS od
                        JOIN :product_table AS p
                            ON od.product_id = p.product_id
                        WHERE od.order_id = $*
                        ORDER BY od.label ASC
                    )
                    SELECT :order_alias_o,
                        array_agg(d) AS details
                    FROM :order_table AS o
                    JOIN details AS d
                        ON o.order_id = p.order_id
                    WHERE o.order_id = $*
                    GROUP BY :invoice_group_by';
                    
            [...]
        }
        
    }

We would get `product` datas on orderDetail virtualField on primary order virtualField.
With modification on this PR it works.