How to set locale default_url_options for functional tests (Rails)

For Rails 5, I found this simple solution In test_helper.rb based on action_dispatch/testing/integration.rb

module ActionDispatch::Integration
  class Session
    def default_url_options
      { locale: I18n.locale }
    end
  end
end

In the Rails 3.1-stable branch, the process method is now within a Behavior module. So here is the code which worked for me (slightly different from John Duff's answer):

class ActionController::TestCase

  module Behavior
    def process_with_default_locale(action, parameters = nil, session = nil, flash = nil, http_method = 'GET')
      parameters = { :locale => I18n.default_locale }.merge( parameters || {} )
      process_without_default_locale(action, parameters, session, flash, http_method)
    end 
    alias_method_chain :process, :default_locale
  end 
end

And I made sure this code gets called before running the specs/tests. A good place to put it is in the test_helper class.