While links to generated HTML from RDoc file needs to be prefixed
by "rdoc-ref:" currently, in case of explicit references this
seems just redundant.
Also GitHub RDoc support does not work with this prefix.
This patch lets links to such document texts (".rb", ".rdoc" and
".md" now) refer URLs generated by `RDoc::TopLevel#http_url`
without the prefix.
https://github.com/ruby/rdoc/commit/f18b27b69d