Simplify association decoration

We don't actually need to reflect on the association because we have
already fetched the associated object - we can just infer the
appropriate decorator from it directly.

lib/draper/decorated_association.rb

@@ -47,21 +47,9 @@ module Draper
         options[:with] = :infer
         Draper::CollectionDecorator
       else
-        "#{association_class}Decorator".constantize
+        undecorated.decorator_class
       end
     end
 
-    def association_class
-      if !options[:polymorphic] && association_reflection
-        association_reflection.klass
-      else
-        undecorated.class
-      end
-    end
-
-    def association_reflection
-      @reflection ||= source.class.reflect_on_association(association) if source.class.respond_to?(:reflect_on_association)
-    end
-
   end
 end