PYTHON-2680 Fix test_bad_dbref due to $ref/$id changes

Author: ShaneHarvey

test/test_bson.py

@@ -490,8 +490,7 @@ def test_bad_dbref(self):
         ref_only = {'ref': {'$ref': 'collection'}}
         id_only = {'ref': {'$id': ObjectId()}}
 
-        self.assertEqual(DBRef('collection', id=None),
-                         decode(encode(ref_only))['ref'])
+        self.assertEqual(ref_only, decode(encode(ref_only)))
         self.assertEqual(id_only, decode(encode(id_only)))
 
     def test_bytes_as_keys(self):