instantiate_builder

# File actionpack/lib/action_view/helpers/form_helper.rb, line 951
        def instantiate_builder(record_name, record_object, options, &block)
          case record_name
          when String, Symbol
            object = record_object
            object_name = record_name
          else
            object = record_name
            object_name = ActiveModel::Naming.param_key(object)
          end

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