 'in' implementation for ForeignLists does not handle None

Given a model with the field:

``` python
    logs = fields.ForeignField('nodelog', list=True, backref='logged')
```

The `in` operator doesn't return the correct value if `None` is in the list.

``` python
node.logs # => [<NodeLog...>, None]
None in node.logs # => False
```

The problem seems to lie in the `__contains__` implementation of ForeignLists.
