Merge branch 'master' of github.com:tbfenet/incubator-spark

PartitionPruningRDD is using index from parent

I was getting a ArrayIndexOutOfBoundsException exception after doing union on pruned RDD. The index it was using on the partition was the index in the original RDD not the new pruned RDD.

(cherry picked from commit 2fead510f74b962b293de4d724136c24a9825271)
Signed-off-by: Reynold Xin <rxin@apache.org>