Fix date parsing to match docs and support timestamps

[pgjones]

The yup documentation states

The default cast logic of date is pass the value to the Date
constructor, failing that, it will attempt to parse the date as an
ISO date string.

which wasn't true and would manifest for timestamps i.e.

const schema = yup.date();
schema.validateSync(1411500325000) // Failed to validate

as the cast logic would try Date.parse(1411500325000) which would return an invalid date.