[SPARK-11980][SPARK-10621][SQL] Fix json_tuple and add test cases for
[SPARK-11980][SPARK-10621][SQL] Fix json_tuple and add test cases for

Added Python test cases for the function `isnan`, `isnull`, `nanvl` and `json_tuple`.

Fixed a bug in the function `json_tuple`

rxin , could you help me review my changes? Please let me know anything is missing.

Thank you! Have a good Thanksgiving day!

Author: gatorsmile <gatorsmile@gmail.com>

Closes #9977 from gatorsmile/json_tuple.

(cherry picked from commit 068b6438d6886ce5b4aa698383866f466d913d66)
Signed-off-by: Reynold Xin <c9d35a384bc579d1c6c8c055f2a2b5c04e11b521@databricks.com>

haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark