SPARK-1019: pyspark RDD take() throws an NPE

Author: Patrick Wendell <pwendell@gmail.com>

Closes #112 from pwendell/pyspark-take and squashes the following commits:

daae80e [Patrick Wendell] SPARK-1019: pyspark RDD take() throws an NPE
(cherry picked from commit 4ea23db0efff2f39ac5b8f0bd1d9a6ffa3eceb0d)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>