Throw error in replacer if date is invalid

[orhoj]

Purpose

Prevent an issuer from trying to add an invalid date to the wallet.

Changes

• Throw an error in the JSON replacer if the Date is not valid.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.