在Rails中测试时如何设置HTTP_REFERER？

test "should redirect to referer" do
  get :index, params: { referer: "http://example.com" }
  assert_redirected_to "http://example.com"
end

class MyController< ApplicationController
  def index
    if params[:referer]
      request.env["HTTP_REFERER"] = params[:referer]
    end
    # ...
  end
end