[SPARK-15520][SQL] SparkSession builder in python should also allow overriding...

[SPARK-15520][SQL] SparkSession builder in python should also allow overriding confs of existing sessions

## What changes were proposed in this pull request?

This fixes the python SparkSession builder to allow setting confs correctly. This was a leftover TODO from https://github.com/apache/spark/pull/13200.

## How was this patch tested?

Python doc tests.

cc andrewor14

Author: Eric Liang <ekl@databricks.com>

Closes #13289 from ericl/spark-15520.