Consider a special case for type Union option with an unknown case

[NinoFloris]

I'd like to propose a special case for the type signature where an option wraps a union.

I believe that when external input is not correct (say an unknown case) that if the type is Union option it should be a default or a config option to return None instead of throwing

Currently an unknown case will just throw in the recursive call to the same serializer with
System.Exception: Cannot parse DU field-less value: z. Expected names: x, y

[NinoFloris]

I'm currently using this as a converter inserted before CompactUnionConverter.

https://gist.github.com/NinoFloris/327c4b4af2c09b159b4af85d58eee4e8

(I have just licensed it under CC0, so it's public domain!)

[blumu]

@NinoFloris Great suggestion. If you could turn your gist into a pull request we could consider integrating in into the official FSharpLu.