How to test actions that redirect to request.referer

describe "POST #add_user" do
  it "adds a new user to the selected plan" do
    @request.env['HTTP_REFERER'] = root_url  # <<<<--------!
    plan = FactoryGirl.create :plan
    expect {
      post :add_user, {:id => plan.to_param, :user => {:id => keeper.to_param}}, valid_session
    }.to change(plan.users, :count).by(1)
  end
end