Fix _.shuffle on small collections. [closes #609]

lodash.js

@@ -5242,19 +5242,16 @@
     function shuffle(collection) {
       collection = toIterable(collection);
 
-      var index = 0,
+      var index = -1,
           length = collection.length,
           result = Array(length);
 
-      if (length) {
-        result[index] = collection[index];
-      }
       while (++index < length) {
-        var value = collection[index],
-            rand = baseRandom(0, index - 1);
-
-        result[index] = result[rand];
-        result[rand] = value;
+        var rand = baseRandom(0, index);
+        if (index != rand) {
+          result[index] = result[rand];
+        }
+        result[rand] = collection[index];
       }
       return result;
     }

test/test.js

@@ -8907,9 +8907,12 @@
       deepEqual(_.shuffle(object).sort(), array);
     });
 
-    test('should shuffle an object', 1, function() {
-      var actual = _.shuffle(object);
-      deepEqual(actual.sort(), array);
+    test('should shuffle small collections', 1, function() {
+      var actual = _.times(1000, function() {
+        return _.shuffle([1, 2]);
+      });
+
+      deepEqual(_.sortBy(_.uniq(actual, String), '0'), [[1, 2], [2, 1]]);
     });
 
     test('should treat number values for `collection` as empty', 1, function() {