serializing an object with an attribute set to `None`

e.g. 
```python 
user = get_user() # <User(foo=None)>
user.serialize(include=('foo',)) # ValueError()
```

but what we want (IMO):
```python 
user = get_user() # <User(foo=None)>
user.serialize(include=('foo',)) # { 'foo': None }
```