Fix belongs_to cmodel inheritance.

While has_many was still working as intended, some changes to
belongs_to broke a cmodel inheritance use case. This use case was
not adequately tested corrected and I've updated the appropriate
unit test to check for the below.

Given the following setup:

    class SimpleCollection < ActiveFedora::Base
       has_many :objects, property: :is_part_of, class_name: 'Object'
     end

     class ComplexCollection < SimpleCollection
     end

     class Item < ActiveFedora::Base
       belongs_to :collection, :property=>:is_part_of, \
       :class_name=>''SimpleCollection”
     end

Doing the following should work:

    object = Item.new
    object.collection = ComplexCollection.create
    object.save!

    object.collection #returns the complex collection

In our exact use case, we have “SystemCollection” and
“OAICollection” that inherit from a base “Collection”
class. Before this fix, mapping to “Collection” would
return neither despite that being in their cmodel
structure since the code was updated to only look at an
exact class match.

As no one else is using inheritance that I am aware of,
this change should not impact any other hydra heads.
Merge pull request #391 from boston-library/cmodel_inheritance_fix

Fix belongs_to cmodel inheritance.