Tests for DateSerializer and LocaleSerializer and enhacements

- Added tests for the previous pull request
- 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
    + changed wrie method, use writeAscii for language and country fields (not for variant)