[SPARK-7237] Clean function in several RDD methods

Author: tedyu <yuzhihong@gmail.com>

Closes #5959 from ted-yu/master and squashes the following commits:

f83d445 [tedyu] Move cleaning outside of mapPartitionsWithIndex
56d7c92 [tedyu] Consolidate import of Random
f6014c0 [tedyu] Remove cleaning in RDD#filterWith
36feb6c [tedyu] Try to get correct syntax
55d01eb [tedyu] Try to get correct syntax
c2786df [tedyu] Correct syntax
d92bfcf [tedyu] Correct syntax in test
164d3e4 [tedyu] Correct variable name
8b50d93 [tedyu] Address Andrew's review comments
0c8d47e [tedyu] Add test for mapWith()
6846e40 [tedyu] Add test for flatMapWith()
6c124a9 [tedyu] Clean function in several RDD methods