Avoid uninitialized variable warning.

actionpack/lib/action_view/helpers/date_helper.rb

@@ -923,6 +923,7 @@ def to_datetime_select_tag(options = {}, html_options = {})
       private
         def datetime_selector(options, html_options)
           datetime = value(object) || default_datetime(options)
+          @auto_index ||= nil
 
           options = options.dup
           options[:field_name]           = @method_name