Pass the model to the security context when checking credentials.

Hello,

With this small change, the ProcessHandler pass the model to the isGranted method of the security context. This would allow either to use Symfony ACL system or to implement model-based authorization checks in a custom Voter. I have to do this in one of my project and I would prefer have this solution than simulating it with a custom pre-validation listener.

Regards,
Adirelle.

It's a good idea but this will work only if your entity directly implements `ModelInterface`. If someone create a model classe to wrap the entity (as in the [doc](https://github.com/lexik/LexikWorkflowBundle#model-object)) it may not work properly.
A way to solve this could be to add a `getWorkflowSubject()` method in `ModelInterface` to get the final object, so we could do `$this->security->isGranted($roles, $model->getWorkflowSubject())` :smiley: