Add tests around object circular reference issue

[nathandem]

Hi, while trying to find out about where the casting bug comes from I noticed that there's an issue around object circular reference. It was hidden until now because the returned values where not inspected.

This issue seems less of a priority than the other one, but I report it anyway :)

[vlucas]

Thank you for the report and the failing test. This helps me look into it easier :)

[restyler]

I can confirm that validateSync() modifies the validated object if it has nesting, adding Circular ref.
I had to use JSON.parse(JSON.stringify(data)) to do deep clone of input and prevent it.

const schema = v.object({
    "search": v.string().required(),
    "list_date": v.object({
        min: v.string().regex(...regexDateRule)
    }),
    "sort": v.object({
        list_date: v.string().rule(...ascOrDescRule)
    })
});