code: public Proxy setAuthentication( Authentication auth )
      {
          if ( eq( this.auth, auth ) )
          {
              return this;
          }
          return new Proxy( type, host, port, auth );
      }
docs: Sets the authentication to use for the proxy connection.
  
  @param auth The authentication to use, may be {@code null}.
  @return The new proxy, never {@code null}.