Enable fetching of nested polymorphic relations.

[atrauzzi]

If you look at the following method: https://github.com/atrauzzi/authoritaire/blob/master/src/Atrauzzi/Authoritaire/Model/Role.php#L28

I have to implement a workaround to retrieve instances of my polymorphic relation via the join model. I keep getting the message "Class name must be a valid object or a string" when I try to use:

return $this
    ->memberships()
    ->with('authorizable')
    ->get()
;

More importantly, my having to do this is as a result of being unable to automatically join across a join table with a half-polymorphic many-to-many relation: #1922

[taylorotwell]

Eager loading a polymorphic relation from the morphed model is not supported.