Returns the proper response body based on the rquest for 401.

lib/devise/strategies/http_authenticatable.rb

@@ -14,7 +14,7 @@ module Devise
         if resource = mapping.to.authenticate_with_http(username, password)
           success!(resource)
         else
-          custom!([401, custom_headers, ["HTTP Basic: Access denied.\n"]])
+          custom!([401, custom_headers, [response_body]])
         end
       end
 
@@ -28,6 +28,12 @@ module Devise
         ActiveSupport::Base64.decode64(request.authorization.split(' ', 2).last || '')
       end
 
+      def response_body
+        body   = "HTTP Basic: Access denied."
+        method = :"to_#{request.format.to_sym}"
+        {}.respond_to?(method) ? { :error => body }.send(method) : body
+      end
+
       def custom_headers
         {
           "Content-Type" => request.format.to_s,

test/integration/http_authenticatable_test.rb

@@ -20,6 +20,7 @@ class HttpAuthenticationTest < ActionController::IntegrationTest
     sign_in_as_new_user_with_http("unknown", "123456", :xml)
     assert_equal 401, status
     assert_equal "application/xml", headers["Content-Type"]
+    assert response.body.include?("<error>HTTP Basic: Access denied.</error>")
   end
 
   test 'returns a custom response with www-authenticate and chosen realm' do