[FIX] do not load defaults when testing SparkConf in pyspark

The default constructor loads default properties, which can fail the test.

Author: Xiangrui Meng <meng@databricks.com>

Closes #775 from mengxr/pyspark-conf-fix and squashes the following commits:

83ef6c4 [Xiangrui Meng] do not load defaults when testing SparkConf in pyspark