Problems concerning validating collections

Hi,

I've run into some problems while PATCHing collections. The problems are as follows:

When using a *CollectionInputFilter*, the *ContentValidationListener* sets the validation group on the array_keys of the submitted $data. This will usually be an array of items, which result in the keys being 0, 1, 2 etc. This results in an *InputFilterInvalidArgumentException*, because the fields 0, 1, 2 etc aren't defined.

I solved this by extending the *CollectionInputFilter* to allow the array keys of its items to be used as inputs. I also had to override the *setValidationGroup* function, to not set any validationGroup when using the item array keys as validation.

Example:

```php
public function setValidationGroup($name)
{
    if ($this->getUseItemArrayKeysAsValidationGroup()) {
        $this->validationGroup = null;

        return $this;
    }

    return parent::setValidationGroup($name);
}

protected function validateInputs(array $inputs, array $data = array())
{
    if ($this->getUseItemArrayKeysAsValidationGroup()) {
        $inputs = array_keys($data);
    }

    return parent::validateInputs($inputs, $data);
}
```

This solves this problem.