[SPARK-2729][SQL] Added test case for SPARK-2729

This is a follow up of #1636.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #1738 from liancheng/test-for-spark-2729 and squashes the following commits:

b13692a [Cheng Lian] Added test case for SPARK-2729
[SPARK-2797] [SQL] SchemaRDDs don't support unpersist()

The cause is explained in https://issues.apache.org/jira/browse/SPARK-2797.

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1745 from yhuai/SPARK-2797 and squashes the following commits:

7b1627d [Yin Huai] The unpersist method of the Scala RDD cannot be called without the input parameter (blocking) from PySpark.