Fix handling RDoc::Servlet rename to RDoc::RI::Servlet

[junaruga]

This PR fixes #12.

I tested in the cases Ruby 4.1 dev with RDoc::RI::Servlet and Ruby 4.0 with RDoc::Servlet.

---

This commit fixes the following error in Ruby 4.1.0dev.

bin/gem_server.rb

require 'rubygems/server'

gem_server = Gem::Server.new Gem.dir, 8089, false
gem_server.run

$ ruby bin/gem_server.rb
Server started at http://0.0.0.0:8089
Server started at http://[::]:8089
/home/jaruga/.local/ruby-4.1.0-debug-7d4941d3e1-openssl-4.1.0-5e32b3e3fa/lib/ruby/gems/4.1.0+1/gems/rubygems-server-0.3.0/lib/rubygems/server.rb:825:in 'Gem::Server#run': uninitialized constant RDoc::Servlet (NameError)

      @server.mount '/doc_root', RDoc::Servlet, '/doc_root'
                                     ^^^^^^^^^
Did you mean?  RDoc::Server
	from bin/gem_server.rb:4:in '<main>'

Because ruby/rdoc renamed the RDoc::Servlet to RDoc::RI::Servlet at the following commit.

ruby/rdoc@3c6f5f6

[hsbt]

@junaruga Thanks! I fixed some old issue at #15

Could you rebase from the current master branch?

[junaruga]

Could you rebase from the current master branch?

@hsbt Thank you for working on this PR. I rebasd the PR on the current master branch, and tested the above gem_server.rb in the cases Ruby 4.1 dev with RDoc::RI::Servlet and Ruby 4.0 with RDoc::Servlet again. The command log is below.

$ ~/.local/ruby-4.1.0-debug-7d4941d3e1-openssl-4.1.0-5e32b3e3fa/bin/ruby -I ~/git/rubygems/rubygems-server/lib gem_server.rb
Server started at http://0.0.0.0:8089
Server started at http://[::]:8089

$ ~/.local/ruby-4.0.0-debug/bin/ruby -I ~/git/rubygems/rubygems-server/lib gem_server.rb
Server started at http://0.0.0.0:8089
Server started at http://[::]:8089