Changed reduceRight to avoid unnecessarily cloning an array.

`toArray` returns the first argument unmodified if it passes `isArray`.
So if `obj` passes `isArray`, call `slice` on it directly, otherwise use `toArray`.

underscore.js

@@ -110,7 +110,7 @@
       if (context) iterator = _.bind(iterator, context);
       return obj.reduceRight(iterator, memo);
     }
-    var reversed = _.clone(_.toArray(obj)).reverse();
+    var reversed = (_.isArray(obj) ? obj.slice() : _.toArray(obj)).reverse();
     return _.reduce(reversed, iterator, memo, context);
   };