title: Devise: authentication_token column in user model is null
body: <p>I just created a sample application with devise plugin installed. I have <code>:token_authenticatable</code> in my user model but for some reason when I create a user it creates it with <code>authentication_token</code> column as <code>NULL</code>. </p>
  
  <p>It looks like you need to set <code>u.ensure_authentication_token!</code> when creating a new user to generate a token.</p>
  
  <p>Am I missing something or I need to override devise's code?</p>