Add guard check to _.last()

Allows _.last() to work as expected with _.map().

test/arrays.js

@@ -30,6 +30,8 @@ $(document).ready(function() {
     equals(_.last([1,2,3], 2).join(', '), '2, 3', 'can pass an index to last');
     var result = (function(){ return _(arguments).last(); })(1, 2, 3, 4);
     equals(result, 4, 'works on an arguments object');
+    result = _.map([[1,2,3],[1,2,3]], _.last);
+    equals(result.join(','), '3,3', 'works well with _.map');
   });
 
   test("arrays: compact", function() {