[SPARK-12711][ML] ML StopWordsRemover does not protect itself from column name duplication

[grzegorz-chilkiewicz]

Fixes problem and verifies fix by test suite.
Also - adds optional parameter: nullable (Boolean) to: SchemaUtils.appendColumn
and deduplicates SchemaUtils.appendColumn functions.

[jkbradley]

Could you please add tag "[ML]" to the PR title?

[jkbradley]

typo: "outputCol"

[jkbradley]

Also, please add a (short) PR description (in your first PR comment) since that will become part of the commit message.

[SparkQA]

Test build #2377 has finished for PR 10741 at commit ca9a852.

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

[grzegorz-chilkiewicz]

Is everything ok with this PR?
Did I miss something?
Can we merge it with apache/spark ?

[jkbradley]

LGTM
I'll run tests before merging

[SparkQA]

Test build #2461 has finished for PR 10741 at commit 37af391.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[jkbradley]

Just copy one of the datasets from an above test. That should fix the error.

[grzegorz-chilkiewicz]

I missed out that second column in dataSet was totally empty... - and that was the problem...
I do not want to make that example too complicated, because this test does not even check correctness of execution result
I'm sorry for problems

[marmbrus]

ok to test

[jkbradley]

LGTM pending tests

[SparkQA]

Test build #50573 has finished for PR 10741 at commit 49fd362.

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

[jkbradley]

Merging with master and branch-1.6
Thanks for the PR!