Fixed potential wrong $limit parameter in the child cursor if a $limit i...

Fixed potential wrong $limit parameter in the child cursor if a $limit is provided to the parent.

A problem occured when i passed a cursor with a defined limit.

```php
// Parent cursor with a limit
$items = $documentManager->getRepository('...')->findBy(array())->limit(50);

$self = $this;
$this->spork->process(
    $items,
    function($item, $index, $batch, $fifo) use ($self) {
        $self->process($item);
    },
    new DoctrineMongoStrategy(2)
);
```
And i would like to have the following forks:
[0-25[
[25-50[

Thanks for this very useful library !