DateSerializer and LocaleSerializer enhacements and tests

(*) Enhacements to DateSerializer: use identity comparison
(*) Enhacements to LocaleSerializer:
  - removed copy method, as Locale isImmutable
  - reordered and enhacements to the Locale constants search inside the create method
  - optimized write method: use writeAscii for language and country fields (not for variant)
(*) Added tests for DateSerializer and LocaleSerializer