Change to ensure rolling window kwargs are passed into aggregator

Currently, rolling window accepts "Aggregator and aggregation function keyword arguments" but it doesn't pass these into aggregator.aggregate(). Instead it creates and uses a new dictionary called newkwargs:
aggregator.aggregate(rolling_window_data,axis=dimension + 1,**newkwargs)

This update makes newkwargs a copy of kwargs and includes a test.
Please could you move this to lib/iris/tests/unit/cube/test_Cube.py - Test_rolling_window. (And perhaps rename it as just `test_kwargs` since that's the important aspect.)