Cannot (de)serialize objects with optional fields

Parity uses [a custom `BlockDetails`](https://github.com/paritytech/parity-ethereum/blob/7c335e87642b848f61ab3ee7bbff5964b53d6f11/ethcore/db/src/keys.rs#L148) object with a very weird last field.

`is_finalized` is a boolean field with the following representation:
- A value of `True` is represented as a True boolean field: `len(BlockDetails) == 5`
- A value of `False` is represented as nothing: `len(BlockDetails) == 4`

Presumably they wanted to save a little space in the database by simply not saving this field.

I don't think it's possible to create a subclass of `rlp.Serializable` which handles this use-case.