[SPARK-18690][PYTHON][SQL] Backward compatibility of unbounded frames

## What changes were proposed in this pull request?

Makes `Window.unboundedPreceding` and `Window.unboundedFollowing` backward compatible.

## How was this patch tested?

Pyspark SQL unittests.

Please review http://spark.apache.org/contributing.html

 before opening a pull request.

Author: zero323 <zero323@users.noreply.github.com>

Closes #16123 from zero323/SPARK-17845-follow-up.

(cherry picked from commit a9cbfc4f)
Signed-off-by: Reynold Xin <rxin@databricks.com>