Fix JSON serialization of SpeicalTransacationOutcome

[td202]

Purpose

Linked: Concordium/concordium-client#338 Concordium/concordium-wallet-proxy#119

Fix JSON serialization of SpeicalTransacationOutcome. This is needed for the wallet proxy.

Changes

• Use dropWhile isLower instead of drop 3 to account for the new constructors having different prefixes.

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.