Add a NULL check in ReduceKeyReduceSide#equals().
Revert to using RDD#collect() for map joins,
Since Spark can send Task results through the block manager when the Akka frame size is exceeded

- Reverts Shark PR #98
- Relevant commit on the Spark side: apache/incubator-spark@c75eb14
Merge pull request #187 from harveyfeng/mj-bug-swat

Revert to using RDD#collect() for map joins,