Allow Devise redirection to Doorkeeper authorization

doorkeeper.rb

  # config/initializers/doorkeeper.rb
  Doorkeeper.configure do
    resource_owner_authenticator do
      # fail "Please configure doorkeeper resource_owner_authenticator block located in #{__FILE__}"
      # Put your resource owner authentication logic here.
      if signed_in?
        current_user
      else
        # redirect to /oauth/authorize?... after signin
        session[:user_return_to] = request.fullpath
        redirect_to(new_user_session_url)
      end
    end
  end