[SPARK-14605][ML][PYTHON] Changed Python to use unicode UIDs for spark.ml Identifiable

## What changes were proposed in this pull request?

Python spark.ml Identifiable classes use UIDs of type str, but they should use unicode (in Python 2.x) to match Java. This could be a problem if someone created a class in Java with odd unicode characters, saved it, and loaded it in Python.

This PR: Use unicode everywhere in Python.

## How was this patch tested?

Updated persistence unit test to check uid type

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #12368 from jkbradley/python-uid-unicode.