Issue #103, change initial value detection for reduce()

underscore.js

@@ -100,7 +100,7 @@
   // **Reduce** builds up a single result from a list of values, aka `inject`,
   // or `foldl`. Delegates to **ECMAScript 5**'s native `reduce` if available.
   _.reduce = _.foldl = _.inject = function(obj, iterator, memo, context) {
-    var initial = memo !== void 0;
+    var initial = arguments.length > 2;
     if (obj == null) obj = [];
     if (nativeReduce && obj.reduce === nativeReduce) {
       if (context) iterator = _.bind(iterator, context);