[SPARK-14665][ML][PYTHON] Fixed bug with StopWordsRemover default stopwords

## What changes were proposed in this pull request?

The default stopwords were a Java object.  They are no longer.

## How was this patch tested?

Unit test which failed before the fix

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #12422 from jkbradley/pyspark-stopwords.