AUI-1288 - Datatable Property List errors when 'enter' key is pressed when no active cell

Hi Jon,

Attached is an update for https://issues.liferay.com/browse/AUI-1288.

This pull request contains a fix (c10c7e4) for the bug still occurring on header cells, as well as the addition of unit tests.

Please let me know if there are any issues.

Thanks!