...s of CoGroupedRDD and PairRDDFunctions

This also removes an unnecessary tuple creation in cogroup.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #1447 from sryza/sandy-spark-2519-2 and squashes the following commits:

b6d9699 [Sandy Ryza] Remove missed Tuple2 match in CoGroupedRDD
a109828 [Sandy Ryza] Remove another pattern matching in MappedValuesRDD and revert some changes in PairRDDFunctions
be10f8a [Sandy Ryza] SPARK-2519 part 2. Remove pattern matching on Tuple2 in critical sections of CoGroupedRDD and PairRDDFunctions