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

[jkbradley]

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

[SparkQA]

Test build #55941 has finished for PR 12422 at commit e35d482.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[sethah]

@jkbradley It is unclear to me if this was a bug before or if it was by design. I think @holdenk mentioned that there is the benefit of leaving stopwords as a JavaObject so that if the user never accesses them, they don't need to be transferred from Java to a Python list. I don't think the benefit is high since stopwords is short, for now.

Aside from that, it LGTM.

[holdenk]

The original design was to allow us to not have to transfer it if the user was using the default stop words, but overall this change looks good me (I'm not sure I'd call it a bug but that a minor point). That being said the stopword list is small enough this optimization isn't important and this simplifies the work going on in #11939 so LGTM.

[jkbradley]

OK thanks for checking this
Merging with master and branch-1.6