Added a unit test for a re-producable update error.

Author: Xin Dong

test/correctness/smoke/test_update.py

@@ -24,6 +24,20 @@
 from collections import OrderedDict
 import pytest
 
+@pytest.mark.xfail
+def test_update_array_containing_none_value(fixture_collection):
+    collection = fixture_collection
+
+    collection.insert({'A': [1, None]})
+
+    docCount = collection.find( {'A': {'$in': [ None]}}).count()
+    assert docCount == 1, "Expect 1 documents before the update but got {}".format(docCount)
+
+    collection.update({'A': {'$size': 2}} ,  OrderedDict([('$pop', {'A': -1})]))
+
+    docCount = collection.find( {'A': {'$in': [ None]}}).count()
+    assert docCount == 1, "Expect 1 documents after the update but got {}".format(docCount)
+
 def test_addToSet_with_none_value(fixture_collection):
     collection = fixture_collection