ActiveResource with OAuth2

ar_oauth2_example.rb

class ProtectedResource < ActiveResource::Base 
  # since the gateway uses oauth2 every call needs to be authenticated,
  # so the token for is needed
  class << self
    attr_accessor :token_string
  end

  # setup the headers for oauth2
  def self.headers
    { 'authorization' => "Bearer #{token_string}"}
  end
end

How do you set the token_string?