title: undefined local variable or method `current_user' when devise routes are defined within a scope
body: <p>I build a multitenant app and I have specific logic for authentication in two tenants so I defined the devises routes in my constrained scope (in order to have two sets of namespaced routes, with their own controllers):</p>
  
  <pre class="lang-rb prettyprint-override"><code>scope(as: 'geea', module: 'geea', constraints: TenantConstraint.new('geea')) do
    devise_for(:users, {
      encryptor: :md5,
      controllers: {
        confirmations:      'geea/users/confirmations',
        sessions:           'geea/users/sessions',
        registrations:      'geea/users/registrations',
        omniauth_callbacks: 'geea/users/omniauth_callbacks',
        passwords:          'geea/users/passwords'
      }
    })
  
    devise_scope :user do
      namespace :backoffice do
        get    'signin',  to: 'sessions#new',     as: 'new_user_session'
        post   'signin',  to: 'sessions#create',  as: 'user_session'
        delete 'signout', to: 'sessions#destroy', as: 'destroy_user_session'
      end
    end
  end
  </code></pre>
  
  <p>The problem is that causes Devise helpers not to be available in my controllers.</p>
  
  <pre><code>undefined local variable or method `current_user' for #&lt;Geea::Frontoffice::HomeController:0x007fd14d4a36e0&gt;
  </code></pre>
  
  <p>How could I deal with it?</p>
  
  <p>Thanks!</p>