Fix bug where $this->columns is set to null instead of [] if selected columns are empty for case of selecting all columns.

Add missing mDataProp data from DT1.9 data to column data array.
Fix some return values.
Merge pull request #142 from phazei/master-phazei

Fix bug where $this->columns is set to null instead of [] if selected.columns are empty for case of selecting all columns.