HeaderEnricherSpec, RLRS JavaDocs; HES Fixes

- Add JavaDocs for HeaderEnricherSpec, RecipientListRouterSpec.
- Add missing overloaded methods for map-based `header` methods with an `override` parameter.
- Remove unused generic parameter from `headerChannelsToString`
No, we delegate to different method not the same.
we do `headers(headers.get())`, not `headers(headers)`.