[SPARK-12353][STREAMING][PYSPARK] Fix countByValue inconsistent output in Python API

The semantics of Python countByValue is different from Scala API, it is more like countDistinctValue, so here change to make it consistent with Scala/Java API.

Author: jerryshao <sshao@hortonworks.com>

Closes #10350 from jerryshao/SPARK-12353.