[SPARK-12780][ML][PYTHON][BACKPORT] Inconsistency returning value of ML python models' properties
[SPARK-12780][ML][PYTHON][BACKPORT] Inconsistency returning value of ML python models' properties

Backport of [SPARK-12780] for branch-1.6

Original PR for master: https://github.com/apache/spark/pull/10724

This fixes StringIndexerModel.labels in pyspark.

Author: Xusen Yin <yinxusen@gmail.com>

Closes #10950 from jkbradley/yinxusen-spark-12780-backport.

haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark