assign_headers_to_message

# File actionmailer/lib/action_mailer/base.rb, line 891
    def assign_headers_to_message(message, headers)
      assignable = headers.except(:parts_order, :content_type, :body, :template_name,
                                  :template_path, :delivery_method, :delivery_method_options)
      assignable.each { |k, v| message[k] = v }
    end