do not .classed() layers in ensureSingle if className not given

... which is an improvement over commit
    8ca4101

Author: etpinard

src/lib/index.js

@@ -710,7 +710,7 @@ lib.isD3Selection = function(obj) {
  *
  * @param {d3 selection} parent : parent selection of the element in question
  * @param {string} nodeType : node type of element to append
- * @param {string} className : class name of element in question
+ * @param {string} className (optional) : class name of element in question
  * @param {fn} enterFn (optional) : optional fn applied to entering elements only
  * @return {d3 selection} selection of new layer
  *
@@ -737,7 +737,8 @@ lib.ensureSingle = function(parent, nodeType, className, enterFn) {
     var sel = parent.select(nodeType + (className ? '.' + className : ''));
     if(sel.size()) return sel;
 
-    var layer = parent.append(nodeType).classed(className || '', true);
+    var layer = parent.append(nodeType);
+    if(className) layer.classed(className, true);
     if(enterFn) layer.call(enterFn);
 
     return layer;