[SPARK-14772][PYTHON][ML] Fixed Params.copy method to match Scala implementation

## What changes were proposed in this pull request?
Fixed the PySpark Params.copy method to behave like the Scala implementation.  The main issue was that it did not account for the _defaultParamMap and merged it into the explicitly created param map.

## How was this patch tested?
Added new unit test to verify the copy method behaves correctly for copying uid, explicitly created params, and default params.

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #17048 from BryanCutler/pyspark-ml-param_copy-Scala_sync-SPARK-14772-2_1.