return_to redirect fix

Fixed invalid array keys, excess variable removed.

This commit fixes invalid array keys, which were added before. In previous version you were required to use data in such array format: 

```
['User' => 
    'username' => 'value',
    'password' => 'value',
    ['User' => 
        ['return_to' => '/some/url/here']
    ]
]
```
It required such using of FormHelper:

```
$this->Form->hidden('User.User.return_to', array('value' => '/some/url/here'));
```
Instead of:
```
$this->Form->hidden('return_to', array('value' => '/some/url/here'));
//or
$this->Form->hidden('User.return_to', array('value' => '/some/url/here'));
```
I fixed this case and removed `$data` array variable, that was used only for `return_to` key access.