Correctly serialize ListType/DictType items when None

This patch fixes an issue where `ListType` and `DictType` failed to
consult their contained items on whether the items should be
serialized when None.

Before this commit, the attribute indicating whether a None-valued
`ListType/DictType` field should be serialized as part of its
containing model was also applied to items within the list/dict. The
actual attribute relating to the nested items
(`ListType/DictType.field.serialize_when_none`) was ignored.

Additionally, `ListType/DictType.field` did not have its
`owner_model` set and therefore the `allow_none()` check wouldn't
fall back to the correct default (that of the model) even if it were
performed. This is also fixed.
Tests for MultiType.field.serialize_when_none