spec_through_test_controller.rb

class TestController < ApplicationController
  before_action :authenticate_user!
  def custom
    render text: 'response'
  end
end

describe TestController do
  describe "authenticate_user!" do
    context "when user not logged in" do
      before do
        subject.send(:current_user=, nil)
        routes.draw { get 'custom' => 'test#custom' }
        get 'custom'
      end

      it { should respond_with :unauthorized }

      it "returns nothing" do
        expect(response.body).to be_blank
      end
    end
  end
end