[SPARK-2270] Kryo cannot serialize results returned by asJavaIterable

and thus groupBy/cogroup are broken in Java APIs when Kryo is used).

@pwendell this should be merged into 1.0.1.

Thanks @sorenmacbeth for reporting this & helping out with the fix.

Author: Reynold Xin <rxin@apache.org>

Closes #1206 from rxin/kryo-iterable-2270 and squashes the following commits:

09da0aa [Reynold Xin] Updated the comment.
009bf64 [Reynold Xin] [SPARK-2270] Kryo cannot serialize results returned by asJavaIterable (and thus groupBy/cogroup are broken in Java APIs when Kryo is used).
(cherry picked from commit 7ff2c754f340ba4c4077b0ff6285876eb7871c7b)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>