Refactor extract method `token_correct?`
Refactor extract TokenComparator, inject dependency

Add TokenComparator public interface specs
Add TokenComparator private specs

The dependency to the Devise secure comparison method is now encapsulated
in the `ActsAsTokenAuthenticationHandler#token_comparator` method.

If necessary, the `#token_correct?` specs could be written using
a DummyTokenComparator instead of the Devise-dependent TokenComparator.
Refactor extract method `find_record_from_identifier`
Minor refactor rename variables for coherence and clarity