# File lib/httpclient/auth.rb, line 187
    def filter_response(req, res)
      command = nil
      if res.status == HTTP::Status::PROXY_AUTHENTICATE_REQUIRED
        if challenge = parse_authentication_header(res, 'proxy-authenticate')
          uri = req.header.request_uri
          challenge.each do |scheme, param_str|
            @authenticator.each do |auth|
              if scheme.downcase == auth.scheme.downcase
                challengeable = auth.challenge(uri, param_str)
                command = :retry if challengeable
              end
            end
          end
          # ignore unknown authentication scheme
        end
      end
      command
    end