gistfile1.rb

Module.class_eval do
  def temporal_const(name, value)
    const_set(name, value)
    yield
    remove_const(name)
  end
end

in use ...

  context "#base_uri" do
    it "builds a URI:HTTP with a port" do
      MailerHelper.temporal_const 'HOST', "localhost:3000" do
        helper.send(:base_uri).to_s.should == "http://localhost:3000"
      end
    end