code: public PaymillList<Client> list( Client.Filter filter, Client.Order order ) {
      return this.list( filter, order, null, null );
    }
docs: This function returns a {@link List} of PAYMILL {@link Client} objects. In which order this list is returned depends on the
  optional parameters. If <code>null</code> is given, no filter or order will be applied.
  @param filter
  {@link com.paymill.models.Client.Filter} or <code>null</code>
  @param order
  {@link com.paymill.models.Client.Order} or <code>null</code>
  @return {@link PaymillList} which contains a {@link List} of PAYMILL {@link Client}s and their total count.