Rspec issue when testing user model

user.rb

class User < ActiveRecord::Base
  validates :auth_token, uniqueness: true
  before_validation :downcase_email
  before_create :generate_authentication_token!
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable

  def generate_authentication_token!
    begin
      self.auth_token = Devise.friendly_token
    end while self.class.exists?(auth_token: auth_token)
  end

  private
    def downcase_email
      self.email = self.email.downcase if self.email.present?
    end
end

user_model_spec.rb

require 'rails_helper'

RSpec.describe User, type: :model do
  subject(:user) { FactoryGirl.create(:user) }

  it { should respond_to(:auth_token) }
  it { should validate_presence_of(:email) }
  it { should validate_uniqueness_of(:email).case_insensitive }
  it { should validate_confirmation_of(:password) }
  it { should allow_value('[email protected]').for(:email) }
  it { should validate_uniqueness_of(:auth_token) }

  describe "authentication token should work" do
    let!(:user) { FactoryGirl.create :user }
    
    it "generates a unique token" do
      Devise.stub(:friendly_token).and_return("auniquetoken123")
      user.generate_authentication_token!
      expect(user.auth_token).to eql "auniquetoken123" 
    end

    it "generates another token when one already been taken" do
      existing_user = FactoryGirl.create(:user, auth_token: "auniquetoken123")
      user.generate_authentication_token!
      expect(user.auth_token).not_to eql existing_user.auth_token 
    end
  end
end

user_model_spec_not_working.rb

require 'rails_helper'

RSpec.describe User, type: :model do
  before { @user = FactoryGirl.create(:user) }
  subject { @user } 

  it { should respond_to(:auth_token) }
  it { should validate_presence_of(:email) }
  it { should validate_uniqueness_of(:email).case_insensitive }
  it { should validate_confirmation_of(:password) }
  it { should allow_value('[email protected]').for(:email) }
  it { should validate_uniqueness_of(:auth_token) }

  describe "#{generate_authentication_token!}" do
    before { @user = FactoryGirl.create(:user) }
    subject { @user } 

    it "generates a unique token" do
      Devise.stub(:friendly_token).and_return("auniquetoken123")
      @user.generate_authentication_token!
      expect(@user.auth_token).to eql "auniquetoken123" 
    end

    it "generates another token when one already been taken" do
      existing_user = FactoryGirl.create(:user, auth_token: "auniquetoken123")
      @user.generate_authentication_token!
      expect(@user.auth_token).not_to eql existing_user.auth_token 
    end
  end
end