[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.

python/pyspark/sql/session.py

@@ -138,24 +138,37 @@ class SparkSession(object):
             """Gets an existing :class:`SparkSession` or, if there is no existing one, creates a
             new one based on the options set in this builder.
 
-            This method first checks whether there is a valid thread-local SparkSession,
-            and if yes, return that one. It then checks whether there is a valid global
-            default SparkSession, and if yes, return that one. If no valid global default
-            SparkSession exists, the method creates a new SparkSession and assigns the
-            newly created SparkSession as the global default.
+            This method first checks whether there is a valid global default SparkSession, and if
+            yes, return that one. If no valid global default SparkSession exists, the method
+            creates a new SparkSession and assigns the newly created SparkSession as the global
+            default.
+
+            >>> s1 = SparkSession.builder.config("k1", "v1").getOrCreate()
+            >>> s1.conf.get("k1") == "v1"
+            True
 
             In case an existing SparkSession is returned, the config options specified
             in this builder will be applied to the existing SparkSession.
+
+            >>> s2 = SparkSession.builder.config("k2", "v2").getOrCreate()
+            >>> s1.conf.get("k1") == s2.conf.get("k1")
+            True
+            >>> s1.conf.get("k2") == s2.conf.get("k2")
+            True
             """
             with self._lock:
-                from pyspark.conf import SparkConf
                 from pyspark.context import SparkContext
-                from pyspark.sql.context import SQLContext
-                sparkConf = SparkConf()
+                from pyspark.conf import SparkConf
+                session = SparkSession._instantiatedContext
+                if session is None:
+                    sparkConf = SparkConf()
+                    for key, value in self._options.items():
+                        sparkConf.set(key, value)
+                    sc = SparkContext.getOrCreate(sparkConf)
+                    session = SparkSession(sc)
                 for key, value in self._options.items():
-                    sparkConf.set(key, value)
-                sparkContext = SparkContext.getOrCreate(sparkConf)
-                return SQLContext.getOrCreate(sparkContext).sparkSession
+                    session.conf.set(key, value)
+                return session
 
     builder = Builder()