Phalcon\Mvc\Model\Query\Builder limit bug

There is no way to unset limit.
For example:

``` PHP
$qb = new \Phalcon\Mvc\Model\Query\Builder;
$qb->limit(5);
$qb->limit(null); // expected result is no limit

var_dump($qb->getLimit()); // returns 'int(0)'
```

I belive it's because of casting limit in method to int.
