[SPARK-7683][PYSPARK] Confusing behavior of fold function of RDD in pyspark

Fix order of arguments that Pyspark RDD.fold passes to its op -  should be (acc, obj) like other implementations.

Obviously, this is a potentially breaking change, so can only happen for 2.x

CC davies

Author: Sean Owen <sowen@cloudera.com>

Closes #10771 from srowen/SPARK-7683.