has_many_polymorphs broken for Rails 2.1.x

The guys on has_many_polymorphs are doing a great job keeping the plugin up to date with edge Rails.

This commit broke the plugin with Rails 2.1.x

NameError: uninitialized constant ActiveRecord::Reflection::ClassMethods::ThroughReflection

Just revert to the AssociationReflection instead of ThroughReflection logic and all should be fine.