content views - changing from has_key to blank on several hash checks

This commit contains a few chagnes on the filter rules helper
to check the rule parameters hash using '!blank?' instead
of 'has_key(:somekey)'.  The reason being we want the same
behavior to be applied if the key is empty (e.g. somekey="")
as if it were non-existent.
jeditable datepicker - allow the user to clear the date

This change will let the user 'clear' the date field.  This
is useful, if the user has a date field currently set with
a date (e.g. 4/1/2013) and they want to reset it (e.g. '').