Demonstrate how to stub current_user in controller spec

[eliotsykes]

E.g.

    it "..." do
      user = create(:user)
      foo = instance_double(Foo)
      foo_id = "123"

      allow(controller).to receive(:current_user).and_return(user)
      expect(user.foos).to receive(:find).with(foo_id).and_return(foo)
      expect(foo).to receive(:destroy).and_return(false)

      sign_in(user)

      delete :destroy, id: foo_id

      expect(flash[:error]).to eq("There was an error.")
    end