405 Allow header is removed when using a custom MethodNotAllowed handler

[BoilingSoup]

By default, Chi sends an Allow response header with the allowed methods when handling a 405.
#776

When I use a custom 405 handler the Allow header is no longer present in responses.

r := chi.NewRouter()
r.MethodNotAllowed(notAllowedJSONHandler)

// "Allow" response header is not automatically set any more.

I'm not sure if this is intended behavior, so I'd like to discuss :)

I think it would be a good default to always send the Allow header for 405 responses, even if using a custom handler.

[VojtechVitek]

I think this makes sense. Free to submit a PR.