Singularize association names before camelization

So that irregular multi-word pluralization rules have to be defined only for snake-case strings.

activerecord/lib/active_record/reflection.rb

@@ -449,9 +449,9 @@ module ActiveRecord
         end
 
         def derive_class_name
-          class_name = name.to_s.camelize
+          class_name = name.to_s
           class_name = class_name.singularize if collection?
-          class_name
+          class_name.camelize
         end
 
         def derive_foreign_key