Does not work against httpbin digest-auth endpoint

[HoneyryderChuck]

I've tried to use this gem against httpbin's endpoints, and either the digest header hasn't been properly generated, or I'm failing to see how to set this up:

require "net/http"
require "net/http/digest_auth"

uri = URI.parse "http://httpbin.org/digest-auth/auth/user/pass"
uri.user = "user"
uri.password = "pass"

h = Net::HTTP.new uri.host, uri.port
h.set_debug_output $stderr

req = Net::HTTP::Get.new uri.request_uri

res = h.request req

digest_auth = Net::HTTP::DigestAuth.new
auth = digest_auth.auth_header uri, res["www-authenticate"], "GET"

req = Net::HTTP::Get.new uri.request_uri
req.add_field "Authorization", auth

res = h.request req

puts
puts "passed" if res.code == "200"
puts "failed" if res.code != "200"