Add. Defaults for aggregator `header` and `kwarg` attributed in `aggregator` method.
Add. Defaults for aggregator `header` and `kwarg` attributed in `aggregator` method.

wikimedia/user_metrics,wikimedia/user_metrics,rfaulkner/wikipedia_user_metrics,wikimedia/analytics-user-metrics,rfaulkner/wikipedia_user_metrics,wikimedia/analytics-user-metrics,rfaulkner/wikipedia_user_metrics,wikimedia/user_metrics,wikimedia/analytics-user-metrics