use hash mapping name to class instead of storing just names

This will alternatively resolve the problem with duplicate classes in
development on reload without the need to explicitly reset the registry

lib_static/open_project/acts/registry_methods.rb

@@ -30,9 +30,7 @@ module OpenProject
   module Acts
     module RegistryMethods
       def instance(model_name)
-        class_name = model_name.singularize.camelize
-
-        class_name.constantize if class_names.include?(class_name)
+        models[model_name.singularize.camelize]
       end
 
       def add(*models)
@@ -44,14 +42,14 @@ def add(*models)
             raise ArgumentError.new("Model #{model} does not include #{acts_as_method_name}")
           end
 
-          class_names << model.name
+          self.models[model.name] = model
         end
       end
 
       private
 
-      def class_names
-        @class_names ||= Set.new
+      def models
+        @models ||= Hash.new
       end
     end
   end