Fix UnicodeEncodeError in PySpark saveAsTextFile().

Fixes SPARK-970.
Merge pull request #218 from JoshRosen/spark-970-pyspark-unicode-error

Fix UnicodeEncodeError in PySpark saveAsTextFile() (SPARK-970)

This fixes [SPARK-970](https://spark-project.atlassian.net/browse/SPARK-970), an issue where PySpark's saveAsTextFile() could throw UnicodeEncodeError when called on an RDD of Unicode strings.

Please merge this into master and branch-0.8.