# File lib/mechanize/http/agent.rb, line 518
  def request_auth_digest request, uri, realm, base_uri, iis
    challenge = @digest_challenges[realm]

    user, password, = @auth_store.credentials_for uri, realm.realm
    uri.user     = user
    uri.password = password

    auth = @digest_auth.auth_header uri, challenge.to_s, request.method, iis
    request['Authorization'] = auth
  end