rails_rack_respone.rb

    # Generate a response array consistent with the requirements of the SPEC.
    # @return [Array] a 3-tuple suitable of `[status, headers, body]`
    # which is suitable to be returned from the middleware `#call(env)` method.
    def finish(&block)
      if STATUS_WITH_NO_ENTITY_BODY[status.to_i]
        delete_header CONTENT_TYPE
        delete_header CONTENT_LENGTH
        close
        return [@status, @headers, []]
      else
        if block_given?
          @block = block
          return [@status, @headers, self]
        else
          return [@status, @headers, @body]
        end
      end
    end