[MONDRIAN-1226] The previous commit for 1226 changed the items in the drillthrough select list in certain cases (testDrillThroughMultiPositionCompoundSlicer).

 [MONDRIAN-1226] The previous commit for 1226 changed the items in the drillthrough select list in certain cases (testDrillThroughMultiPositionCompoundSlicer).
  
This was actually due to an inconsistency in how the select list is handled when the tuples in the slicer are multi-position vs. single-position.
For example, a query with "WHERE (Time.[1997].[Q1])" would include time in the drillthrough select list, but "WHERE {Time.[1997].[Q1], Time.[1997].[Q2]}" would not.

This change makes default select list consistent across the two cases.