update strategy for getting column names
update strategy for getting column names

* if there were ever no results, we would fail later on because we
  wouldn't have any column names
* this should be faster too, because we never have to issue repeat
  queries with alternate bounds

moskvax/TileStache,mapzen/TileStache,mapzen/TileStache,moskvax/TileStache,mapzen/TileStache,moskvax/TileStache