Fixing #95 ... Maloptimization of _.keys for sparse arrays.

underscore.js

@@ -501,7 +501,6 @@
   // Retrieve the names of an object's properties.
   // Delegates to **ECMAScript 5**'s native `Object.keys`
   _.keys = nativeKeys || function(obj) {
-    if (_.isArray(obj)) return _.range(0, obj.length);
     var keys = [];
     for (var key in obj) if (hasOwnProperty.call(obj, key)) keys[keys.length] = key;
     return keys;