Allow Number sub-classes to serialize None to colander.null

This brings back behaviour that was intended to be included in #59 and
commit 24f648d but was accidentally
reverted in 513d860.

Closes #204

colander/__init__.py

@@ -1399,7 +1399,7 @@ class Number(SchemaType):
     num = None
 
     def serialize(self, node, appstruct):
-        if appstruct is null:
+        if appstruct in (null, None):
             return null
 
         try: