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 PR #165