🐛 Fix Hyrax::ChangeSet.inspect method

Prior to this, there was no reference to `model_class` meaning that
`Hyrax::ChangeSet.inspect` would raise an exception.

We need the binding of the `resource_class`, which is available during
the creation of the class but not afterwards.  In other words, by using
the `define_singleton_method` we can leverage the provided
`resource_class` as a bound value for the return value of `.inspect`.

app/models/hyrax/change_set.rb

@@ -21,8 +21,8 @@ def self.ChangeSet(resource_class)
 
       ##
       # @return [String]
-      def self.inspect
-        return "Hyrax::ChangeSet(#{model_class})" if name.blank?
+      define_singleton_method :inspect do
+        return "Hyrax::ChangeSet(#{resource_class})" if name.blank?
         super
       end
     end