apply_defaults

# File actionmailer/lib/action_mailer/base.rb, line 910
      def apply_defaults(headers)
        default_values = self.class.default.except(*headers.keys).transform_values do |value|
          compute_default(value)
        end

        headers_with_defaults = headers.reverse_merge(default_values)
        headers_with_defaults[:subject] ||= default_i18n_subject
        headers_with_defaults
      end