SparkXGBClassifier: Update params getting ignored

Hi folks. Is there a way to update a `SparkXGBClassifier`? Some of the other APIs have `process_type`, `updater`, and `refresh_leaf` params but I don't see those on the pyspark API. And if I set those params (along with `xgb_model`) they get ignored and added to `arbitrary_params_dict`.

```
"arbitrary_params_dict: arbitrary_params_dict This parameter holds all of the additional parameters which are not exposed as the the XGBoost Spark estimator params but can be recognized by underlying XGBoost library. It is stored as a dictionary. (default: {}, current: {'process_type': 'update', 'updater': 'refresh', 'refresh_leaf': True})"
```