fix for Collection#group

lib/mongo/collection.rb

@@ -317,8 +317,9 @@ function () {
         var obj = c.next();
 
         var key = {};
-        for (var i in keys) {
-            key[keys[i]] = obj[keys[i]];
+        for (var i = 0; i < keys.length; i++) {
+            var k = keys[i];
+            key[k] = obj[k];
         }
 
         var aggObj = map.get(key);

test/test_db_api.rb

@@ -607,6 +607,12 @@ class DBAPITest < Test::Unit::TestCase
 
     assert_equal 3, test.group([], {}, {"count" => 0}, "function (obj, prev) { prev.count++; }")[0]["count"]
     assert_equal 1, test.group([], {"a" => {"$gt" => 1}}, {"count" => 0}, "function (obj, prev) { prev.count++; }")[0]["count"]
+
+    test.insert("a" => 2, "b" => 3)
+    expected = [{"a" => 2, "count" => 2},
+                {"a" => nil, "count" => 1},
+                {"a" => 1, "count" => 1}]
+    assert_equal expected, test.group(["a"], {}, {"count" => 0}, "function (obj, prev) { prev.count++; }")
   end
 
   def test_deref