Allows to use a two level array data on 'data' parameter for Form->postLink()

This allow to make use of 

```php
echo $this->Form->postLink( " Export to PDF", array( 'action' => 'imprimir.pdf' ),array( 'data' => $this->Paginator->params() ) );
and recive an array such as:
array(
	'page' => '1',
	'current' => '127',
	'count' => '127',
	'prevPage' => '',
	'nextPage' => '',
	'pageCount' => '1',
	'order' => array(
		'Gasto' => array(
			'fecha' => 'desc'
		)
	),
	'limit' => '10000',
	'paramType' => 'named'
)
```

on the action.
Before the change, the order field throw a warning about converting array to string.