S.optional(S.union(...)) throws

[chenglou]

import * as S from 'sury'                                                                                                                                          
const schema = S.optional(S.union(['a', 'b', 'c']), 'a')
console.log(S.parseOrThrow(undefined, schema))

Works on 10.0.4, throws on 11.0.0-alpha.4

Which is ok if this is the new behavior, but it doesn't throw for other types such as S.optional(S.number)

[DZakh]

There's a regression in later versions. I'm working on a PR to fix it #152