RSpecによるFacebookのOmniAuthログインのテスト

Question

http://qiita.com/mnishiguchi/items/3d6a4ec36c2237a11660

こちらのような記事を参考にして、omniauthのコントローラのテストを書きました。

require 'rails_helper'

RSpec.describe Users::OmniauthCallbacksController, type: :controller do
  include OmniauthUtils
  let(:user) { build(:user) }
  before { Rails.application.env_config["omniauth.params"] = {'user_type' => 'host'} }

  context 'before signup' do
    it 'can signup with facebook' do
      set_omniauth('[e-mail]')
      visit user_facebook_omniauth_callback_path
      expect(page).to have_selector 'div.alert', text: I18n.t('devise.omniauth_callbacks.success', kind: 'Facebook')
    end
  end
end

すると

expected to find css "div.alert" with text "Successfully authenticated from Facebook account." but there were no matches. Also found "×Could not authent
icate you from Facebook because \"No authorization code\".", which matched the selector but not all filters.

のようなエラーが出てしまいます。
authorization codeをどう設定すればいいのかわからず、
そもそも直接user_facebook_omniauth_callback_pathにアクセスするこのテストが正しいのかも怪しいです。わかる方がいらしたら教えていただきたいです。

追記:

  def set_omniauth(email)
    OmniAuth.config.mock_auth[:facebook] = OmniAuth::AuthHash.new({
      "provider" => "facebook",
      "uid"  => "#{email}",
      "credentials" => {
        "token"  => "mock_credentials_token",
        "secret" => "mock_credentials_secret"
      }
    })
  end

def login(object)
    @request.env['devise.mapping'] = Devise.mappings[object.class.name.underscore.to_sym]
    sign_in object
  end

  def login_user
    user = create(:user)
    login(user)
    user
  end