instantiate_builder

# File actionview/lib/action_view/helpers/form_helper.rb, line 1550
        def instantiate_builder(record_name, record_object, options)
          case record_name
          when String, Symbol
            object = record_object
            object_name = record_name
          else
            object = record_name
            object_name = model_name_from_record_or_class(object).param_key if object
          end

          builder = options[:builder] || default_form_builder_class
          builder.new(object_name, object, self, options)
        end