[SPARK-8038] [SQL] [PYSPARK] fix Column.when() and otherwise()

Thanks ogirardot, closes #6580

cc rxin JoshRosen

Author: Davies Liu <davies@databricks.com>

Closes #6590 from davies/when and squashes the following commits:

c0f2069 [Davies Liu] fix Column.when() and otherwise()

(cherry picked from commit 605ddbb2)
Signed-off-by: Reynold Xin <rxin@databricks.com>