Add guard check to _.last()

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

underscore.js

@@ -307,9 +307,9 @@
   };
 
   // Get the last element of an array. Passing **n** will return the last N
-  // values in the array.
-  _.last = function(array, n) {
-    return (n != null) ? slice.call(array, array.length - n) : array[array.length - 1];
+  // values in the array. The **guard** check allows it to work with `_.map`.
+  _.last = function(array, n, guard) {
+    return (n != null) && !guard ? slice.call(array, array.length - n) : array[array.length - 1];
   };
 
   // Trim out all falsy values from an array.