Fix ActiveFedora::Inheritance.base_class for deep File descendants

[mbklein]

Fixes and adds tests for the following issue:

Given:

class A < ActiveFedora::File
end

class B < A
end

Expected:

> A.base_class
=> ActiveFedora::File
> B.base_class
=> ActiveFedora::File

Actual:

> A.base_class
=> ActiveFedora::File
> B.base_class
ActiveFedora::ActiveFedoraError: B doesn't belong in a hierarchy descending from ActiveFedora
from /Users/mbk836/Workspace/hydra/active_fedora/lib/active_fedora/inheritance.rb:18:in `base_class'