I think here is not the best place to decide what hydrator we should use.

What about inside `CreateProduct`:
```php
$this->add(array(
    'name'     => 'product',
    'type'     => 'Application\Form\ProductFieldset',
    'hydrator' => 'classmethods',
    'options' => array(
        'use_as_base_fieldset' => true,
    ),
));
```