title: Rails: NoMethodError in RailsAdmin::MainController#dashboard
body: <p>I was setting up authorization using the code below and got an error. I'm using devise</p>
  
  <pre><code>undefined method `is_admin?' for #&lt;User:0x007f803734ba48&gt;
  </code></pre>
  
  <p>I've already setup the admin user in the console but I'm having issues testing out users trying to log in. </p>
  
  <pre><code>    RailsAdmin.config do |config|
        config.authorize_with do
          redirect_to main_app.root_path unless warden.user.is_admin?
        end
      end
  </code></pre>
  
  <p><a href="https://github.com/sferik/rails_admin/wiki/Authorization" rel="nofollow">https://github.com/sferik/rails_admin/wiki/Authorization</a></p>