Respond to all formats if none is specified.

lib/devise/controllers/internal_helpers.rb

@@ -16,6 +16,7 @@ module Devise
         helper_method *helpers
 
         prepend_before_filter :is_devise_resource?
+        respond_to *Mime::SET.map(&:to_sym) if mimes_for_respond_to.empty?
 
         Devise.routes_prepare do
           skip_before_filter *Devise.mappings.keys.map { |m| :"authenticate_#{m}!" }

test/rails_app/app/controllers/application_controller.rb

@@ -3,8 +3,6 @@
 
 class ApplicationController < ActionController::Base
   protect_from_forgery
-  respond_to :html, :xml, :json
-
   before_filter :current_user
   before_filter :authenticate_user!, :if => :devise_controller?
 end