Date rule string or array

```
 $val->addField('date2')
            ->date('d-m-Y');
```

I`m get error "Illegal string offset 'format'";  in /vendor/fuelphp/validation/src/Rule/Date.php at line 47

Maybe you should use 

```
 $val->addField('date2')
            ->date(['format' =>'d-m-Y']);
```

correct documentation, or correct the problem in the class Date.php $format = $parameters['format'];
