Added forced array type on query_methods parameter

It took me a while to figure out what's wrong, had:
```php
$this->setValidator('xxx', new sfValidatorPropelChoice(array(
    // (...)
    'query_methods' => 'someQueryMethod',
)));
```
When it should be:
```php
$this->setValidator('xxx', new sfValidatorPropelChoice(array(
    // (...)
    'query_methods' => array('someQueryMethod'),
)));
```
Now both examples will work.