[SPARK-7512] [SPARKR] Fix RDD's show method to use getJRDD

Since the RDD object might be a Pipelined RDD we should use `getJRDD` to get the right handle to the Java object.

Fixes the bug reported at
http://stackoverflow.com/questions/30057702/sparkr-filterrdd-and-flatmap-not-working

cc concretevitamin

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #6035 from shivaram/sparkr-show-bug and squashes the following commits:

d70145c [Shivaram Venkataraman] Fix RDD's show method to use getJRDD Fixes the bug reported at http://stackoverflow.com/questions/30057702/sparkr-filterrdd-and-flatmap-not-working