Remove spurious whitespace from class name list before splitting to e…

…liminate null strings, which generate an error from classed(value, false). Fixes #675.

src/core/selection-classed.js

@@ -1,5 +1,5 @@
 d3_selectionPrototype.classed = function(name, value) {
-  var names = name.split(d3_selection_classedWhitespace),
+  var names = d3_collapse(name).split(" "),
       n = names.length,
       i = -1;
   if (arguments.length > 1) {
@@ -11,8 +11,6 @@ d3_selectionPrototype.classed = function(name, value) {
   }
 };
 
-var d3_selection_classedWhitespace = /\s+/g;
-
 function d3_selection_classed(name, value) {
   var re = new RegExp("(^|\\s+)" + d3.requote(name) + "(\\s+|$)", "g");