Add a failing test case for nullary options embedded in some other type

Because decoding a `None` does not consume the nil-byte used to represent
it: any value following a None-variant will be corrupted. (Namely it will
have a leading nil byte on the front.)
Consume the nil byte if an option is nullary
Merge pull request #43 from drbawb/feature/fix-option-nullary

Properly decode `None` variants