[SQL] Python JsonRDD UTF8 Encoding Fix

Only encode unicode objects to UTF-8, and not strings

Author: Ahir Reddy <ahirreddy@gmail.com>

Closes #1914 from ahirreddy/json-rdd-unicode-fix1 and squashes the following commits:

ca4e9ba [Ahir Reddy] Encoding Fix

(cherry picked from commit fde692b361773110c262abe219e7c8128bd76419)
Signed-off-by: Michael Armbrust <michael@databricks.com>