Virtual fields aren't cast

[Blue3957]

I come forward with an issue I mentioned on Slack a couple of days ago.

When setting up relations through virtual fields, using the function cast() leaves them as they are and you end up with something that looks like this:

^ array:6 [▼
  "one_property" => "lorem"
  "virtual_two" => Models\Two {#17 ▶}
  "virtual_three" => DB\CortexCollection {[#26 ▶}
  "two" => array:3 [▶]
  "three" => array:1 [▶]
  "_id" => 1
]

If your virtual function returns an instance of Cortex or CortexCollection, I think it would be desirable to see them get cast when casting the model, just like regular relations are.

An example of a use case would be a virtual field for unpaid invoices from a client. This cannot be setup in the fieldConf as you don't want to get all invoices regardless of status, only the unpaid ones.

Here's a small repo (I made it as concise as possible) that reproduces the error, should you need it.