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.
It's a minor point, but because this test is all about the handling of kwargs I'd be inclined to pass the window as a positional argument. That would help to make `mdtol` stand out.

```python
window = 2
res_cube = self.cube.rolling_window('longitude', iris.analysis.MEAN, window, mdtol=0)
```