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`
The target `HeaderEnricher#setDefaultOverwrite` does not have any JavaDocs. 
Please, add some description here, at least.