[SPARK-6660][MLLIB] pythonToJava doesn't recognize object arrays

davies

Author: Xiangrui Meng <meng@databricks.com>

Closes #5318 from mengxr/SPARK-6660 and squashes the following commits:

0f66ec2 [Xiangrui Meng] recognize object arrays
ad8c42f [Xiangrui Meng] add a test for SPARK-6660