[5.x] Prevent error when cascadeContent is an Eloquent Model

[duncanmcclean]

This pull request fixes an issue when passing an Eloquent Model into a view's `->cascadeContents() method.

I'm currently fixing a bug in Runway, with its frontend routing feature, where the page variable isn't available in Blade views (see statamic-rad-pack/runway#590).

The page variable is only set when the view has cascadeContent set, whereas previously we were only passing an array of the augmented model into the View::with() method.

However, when passing the model to View::cascadeContents(), it resulted in an error because the model doesn't have an id method on it.

I could add an ID method but that has caused issues in the past where folks have already implemented their own id methods.

Ideally, for Eloquent Models, it'd use the model's key anyway.

Anyways, feel free to reject if you don't want this in Core and I'll try and find another way around it.