Fix warning: method redefined; discarding old fields

Follow up to rails#26976.

Author: kamipo

actionview/lib/action_view/helpers/form_helper.rb

@@ -1619,7 +1619,7 @@ def initialize(object_name, object, template, options)
         @index = options[:index] || options[:child_index]
       end
 
-      (field_helpers - [:label, :check_box, :radio_button, :fields_for, :hidden_field, :file_field]).each do |selector|
+      (field_helpers - [:label, :check_box, :radio_button, :fields_for, :fields, :hidden_field, :file_field]).each do |selector|
         class_eval <<-RUBY_EVAL, __FILE__, __LINE__ + 1
           def #{selector}(method, options = {})  # def text_field(method, options = {})
             @template.send(                      #   @template.send(