/**
  * If proxy needs authentication, specify User.
  *
  * @param proxyUser Username for authentication.
  * @return the current {@link Builder} instance
  */
 public Builder proxyUser(String proxyUser) {
   if (proxy == null) {
     proxy = new ProxyConfig(Proxy.Type.HTTP);
   }
   proxy.setProxyUser(proxyUser);
   return this;
 }