Authorization header using ActiveResouce

I'm trying to use ActiveResource in a ruby script to consume a web
service that requires simple HTTP authentication. From the docs, this
seems like it should be trivial:

require 'rubygems'
require 'activeresource' #I'm on 2.1.0

class List < ActiveResource::Base
    self.logger = STDOUT = ''

But when I try to use the model, it throws back a 401--even though the
same resource works just fine when invoked from a web browser. When I
looked at the outbound traffic using WireShark, the Authorization
header isn't even there.

Any hints/suggestions?