CStatElement with join bugfix

Model stat-relation defined with ambigous fields:
```php 
public function relations() {
	return array(
    	'someRelation' => array(
        	self::STAT,
            'SomeModel',
            'primary_id',            
            'join' => "JOIN `another_table` AS `r` ON `r`.`primary_id` = `t`.`primary_id`"
     	),
	);
}
```
throws exception: `Integrity constraint violation: 1052 Column 'primary_id' in field list is ambiguous. The SQL statement executed was:`
```sql
SELECT `primary_id` AS `c`, COUNT(*) AS `s` 
FROM `task` `t` 
JOIN `another_table` AS `r` ON `r`.`primary_id` = `t`.`primary_id`  
WHERE (`t`.`primary_id`='2560') 
GROUP BY `primary_id`
```