Set label text without having to create array 'text' key.

When creating horizontal forms the input default settings specify the label class. Therefore the 'quick setting' of the label text isn't possible anymore (array merge will fail), for example:
```php
 echo $this->Form->input('birth_dt', array(
    'label' => 'Date of birth',
 ));
```
So we needed to do the following:

```php
 echo $this->Form->input('birth_dt', array(
    'label' => array(
         'text' => 'Date of birth'
     ),
 ));
```
This is a little fix so we can set the label again using the 'quick style' without specifying the text key