user_mailer.rb

class UserMailer < ActionMailer::Base
  def signup_notification(user)
    setup_email(user)
    @subject    += "One final step:  please activate." 
    @body[:url]  = "http://#{user.site.domain}/activate/#{user.activation_code}"
    @body[:code] = user.activation_code
  end
end

user_mailer_test.rb

context "The User Mailer", ActionMailer::TestCase do
  tests UserMailer
  specify "should send a confirmation on signup" do
    response = UserMailer.create_signup_notification(@quentin)
    response.subject.should.be "#{user.site.title}: One final step:  please activate." 
    response.body.should.match "http://#{user.site.domain}/activate/#{user.activation_code}"
  end
end