What happened to Builder::XmlMarkup in Rails 3?

I'm struggling with the atom_feed method in a RoR 3 pr开发者_开发百科oject. The method complains about an

undefined local variable or method `xml'

error so I tried passing the XML builder as an option to the atom_feed (i e atom_feed({:xml => Builder::XmlMarkup.new})) but when doing so I get a NameError: uninitialized constant Builder exception instead.

Does anyone know why I'm getting either the first or the second exception?

 require 'active_support/builder' unless defined?(Builder)