Because you say so, I'll keep 208, 506, 508 and 510 unadded until I get a proof.
Among the uncertain four, I've observed 510 responses returned from some local (Japanese) hosting services, but I'm not sure if they are generated by a widely distributed server/middleware or from the companies' in-house software.
Just for the record, status codes 506 and 510 are listed in Apache's http_protocol.c, while 208 is unlisted and 508 is marked as unused.
This issue was solved with changeset r36068.
Akinori, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.
lib/net/http/responses.rb, lib/webrick/httpstatus.rb: Add HTTP
response codes added in RFCs 2817 and 4918. [ruby-core:45547]
[Feature #6569]
lib/net/http/responses.rb: Rename Net::HTTPMultipleChoice to
Net::HTTPMultipleChoices, leaving the former as alias to the
latter for backward compatibitily. [ruby-core:45547]
[Feature #6569]
lib/net/http/responses.rb: Add comments about unused,
still-in-draft and private extension response codes. [ruby-core:45547] [Feature #6569]