misleading but not dangerous selectAlls

there's only one or zero being selected
select should be paired with append
for #1239

src/heatmap.js

@@ -213,7 +213,7 @@ dc.heatMap = function (parent, chartGroup) {
 
         boxes.exit().remove();
 
-        var gCols = _chartBody.selectAll('g.cols');
+        var gCols = _chartBody.select('g.cols');
         if (gCols.empty()) {
             gCols = _chartBody.append('g').attr('class', 'cols axis');
         }
@@ -230,7 +230,7 @@ dc.heatMap = function (parent, chartGroup) {
                .attr('x', function (d) { return cols(d) + boxWidth / 2; })
                .attr('y', _chart.effectiveHeight());
         gColsText.exit().remove();
-        var gRows = _chartBody.selectAll('g.rows');
+        var gRows = _chartBody.select('g.rows');
         if (gRows.empty()) {
             gRows = _chartBody.append('g').attr('class', 'rows axis');
         }