Honour value of Timestampable's "disable_updated_at" parameter in SoftDeleteBehavior

The SoftDeleteBehavior does not honour the value of the "disable_updated_at" parameter in the Timestampable behaviour. 

If "disable_updated_at" is set to "true" this results in the delete() method in BaseXXX.php classes being generated with a call to 
```php
$this->keepUpdateDateUnchanged();
```
which result in Fatal Errors when trying to perform a delete due to the method not existing.

I realise that SoftDelete is deprecated but many users are still using it and this tiny fix would be very helpful for us!