Change the way a field is retrieved for fixing multiple field bug.

I found a problem when using FormHelper to create input fields with multiple values.

Here's an example:

```php
<?php for ($i = 0, $n = count($this->request->data['Contact']['others']); $i <= $n; $i++): ?>
	<?php
		echo $this->Form->input('Contact.others.' . $i . '.0', array(
			'label'			=> __('Key'),
			'type'			=> 'text',
		));
	?>
	<?php
		echo $this->Form->input('Contact.others.' . $i . '.1', array(
			'label'			=> __('Value'),
			'type'			=> 'text',
		));
	?>
<?php endfor; ?>
```

Without this patch, cakephp put "required" class in all "Contact.others.n.0" divs, even if I didn't create a "validate" rule for that key.

In fact, when FormHelper->input() calls Helper->field() method to determine the name of the field, that method doesn't strip all the numbers of a multiple field key.
In this case, for all the "Contact.others.n.0" Helper->field() returns "0", and FormHelper->_introspectModel() returns true because of this:

```php
if ($key === 'validates') {
	if (empty($field)) {
		return $this->fieldset[$model]['validates'];
	} else {
		return isset($this->fieldset[$model]['validates'][$field]) ?
			$this->fieldset[$model]['validates'] : null;
	}
}
```

All the inputs get the "required" class because of the empty() returns true.

I think that also any possible validation rule for the "others" field are not considered without this patch, but I'm not really sure.