Handle merging None into lists/dicts

Signed-off-by: martin f. krafft <madduck@madduck.net>
diff --git a/mergers/dict/tests/test_recursive_update.py b/mergers/dict/tests/test_recursive_update.py
index c1e8501..4410b68 100644
--- a/mergers/dict/tests/test_recursive_update.py
+++ b/mergers/dict/tests/test_recursive_update.py
@@ -43,3 +43,8 @@
             assert len(ret['four'][i]) == 1
             for k,v in ret['four'][i].iteritems():
                 assert k == v
+
+    def test_merge_with_none(self):
+        first = {1:2,3:4}
+        ret = self.merger.merge(first, None)
+        assert ret == first