Ensure that `@JsonUnwrapped` works

[cowtowncoder]

One way to support nested objects with flat structure (like CSV) is to use @JsonUnwrapper. But I don't think currenty tests verify that this works. Test coverage should be added.

[cowtowncoder]

First part verified: with no prefixes, manual schema, read/write work fine.

[cowtowncoder]

Had to improve CsvSchema generation in CsvMapper -- not too hard, now works.