Handle variants where attr is not an array but a function pointer. 

Example code that chrashes without this fix: 

```php
    $resolver->setDefaults(array(                                                 
            'error_bubbling' => false,                                                
            'invalid_message' => "Invalid resource selected",                         
            'type' => false,                                                          
                                                                                      
            'allowed_resource_types' => false,                                        
            'required' => false,                                                      
            'attr' => function(Options $options) use ($router) {                      
                return array(                                                         
                'placeholder' => "Search for a movie",                                
                'class' => 'resource_selector auto_complete',                         
                "data-url-all" => ($options['allowed_resource_types'] !== false) ?    
                        $router->generate('admin_resource_query_type', array('type' => $options['allowed_resource_types'])) :
                         $router->generate('admin_resource_query', array())           
                );                                                                    
                                                                                      
            }                                                                         
            )                                                                         
        );     

```
Merge pull request #153 from tarjei/patch-1

Handle variants where attr is not an array but a function pointer.