title: friendly_id slug is not generating on create
body: <p>I have an active record callback on my user model to generate a username if the user didn't specify a username. This would occur when the user would register via Google, Facebook, or any other third party.</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    extend FriendlyId
    friendly_id :username, use: :slugged
  
    before_validation :set_default_username 
  
    private
  
      def set_default_username
        self.username ||= "user#{User.last.id+1}"
      end
  
  end
  </code></pre>
  
  <p>The problem is, in my seeds file, i noticed for one of my users, the slug was not being created, specifically on this case:</p>
  
  <pre><code>User.create(email:"nousername@outlook.com", password: "password")
  </code></pre>
  
  <p>While the username is being created due to set_default_username, the slug is not being created. Is there any way to fix this?</p>