Updates to imports for future pymongo 2.2

tests/document.py

@@ -1,5 +1,6 @@
 import pickle
 import pymongo
+import bson
 import unittest
 import warnings
 
@@ -2222,7 +2223,7 @@ class DocumentTest(unittest.TestCase):
 
         # Test laziness
         self.assertTrue(isinstance(post_obj._data['author'],
-                                   pymongo.dbref.DBRef))
+                                   bson.DBRef))
         self.assertTrue(isinstance(post_obj.author, self.Person))
         self.assertEqual(post_obj.author.name, 'Test User')