Remove the unnecessary branch in _.shuffle()

The non-zero case works properly for 0 as well.

test/speed.js

@@ -67,4 +67,8 @@
     return _.range(1000);
   });
 
+  JSLitmus.test('_.shuffle()', function() {
+    return _.shuffle(numbers);
+  });
+
 })();

underscore.js

@@ -250,13 +250,9 @@
   _.shuffle = function(obj) {
     var shuffled = [], rand;
     each(obj, function(value, index, list) {
-      if (index == 0) {
-        shuffled[0] = value;
-      } else {
-        rand = Math.floor(Math.random() * (index + 1));
-        shuffled[index] = shuffled[rand];
-        shuffled[rand] = value;
-      }
+      rand = Math.floor(Math.random() * (index + 1));
+      shuffled[index] = shuffled[rand];
+      shuffled[rand] = value;
     });
     return shuffled;
   };