Fix missing task when saving configuration

Fixes half of http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32984&start=0

Bug: $this->options[3] may not exist.  If it does not exist, instead of falling through to the default logic it seems that the apply logic in the switch statement is used leading to confusion.

This fixes that issue by ensuring that if the task array option is not available, task is set to the string default so fall through to default logic.