Added Event Subscriber class to enable PATCH methods. To use PATCH it must be subscribed in the FormType::buildForm method  by adding the Event Subscriber PatchSubscriber:

```php
    if($options['method'] === 'PATCH') {
        // Event to prepare the submitted data for a PATCH
        $builder->addEventSubscriber(new PatchSubscriber());
    }
```

The PATCH subscriber changes the form data to deal with not set values as if they were unchanged.