ActiveRecord::Relation#except and #only loses scope extensions

Hi,

I've written a patch to fix a bug in except and only on
ActiveRecord::Relation to lose any extensions added by earlier scopes.
Since making a pull request is erroring on my on github, I've added
the patch to lighthouse too.

This can be added to the Rails 3.0.x branch as well.

https://github.com/iain/rails/commit/a0bd5ba2fa38155ef6c036b256b08414eb9a2af1
https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/6598-activerecordrelationexcept-and-only-loses-scope-extensions

Any feedback is greatly appreciated

Iain