Handle merging None into lists/dicts

Signed-off-by: martin f. krafft <madduck@madduck.net>
diff --git a/mergers/dict/tests/test_update.py b/mergers/dict/tests/test_update.py
index 8e35c53..02702f7 100644
--- a/mergers/dict/tests/test_update.py
+++ b/mergers/dict/tests/test_update.py
@@ -20,3 +20,8 @@
         assert len(ret) == 4
         for k,v in ret.iteritems():
             assert k == v
+
+    def test_merge_with_none(self):
+        first = {1:2,3:4}
+        ret = self.merger.merge(first, None)
+        assert ret == first