def forward
      fail "downstream app not set" unless @app.respond_to? :call
      status, headers, body = @app.call env
      @response.status = status
      @response.body = body
      @response.headers.merge! headers
      nil
    end
def forward
      fail "downstream app not set" unless @app.respond_to? :call
      status, headers, body = @app.call env
      @response.status = status
      @response.body = body
      @response.headers.merge! headers
      nil
    end
Forward the request to the downstream app -- middleware only.