StringIO#gets("\r\n", 10) returns an extra \n

[eregon]

From ruby/net-imap#470 (comment) by @nevans

@eregon here's the issue I saw with IO#gets(CRLF, limit):

#!/usr/bin/env ruby

puts ?=*72
puts "Ruby Engine:   #{RUBY_ENGINE} #{RUBY_ENGINE_VERSION}"

require 'bundler/inline'
gemfile do
  gem "stringio", ">= 3.1.7"
end
require "stringio"
puts "Using StringIO #{StringIO::VERSION}"
io = StringIO.new("123456789\r\n123456789\r\n")
pp stringio: io.gets("\r\n", 10)

r, w = IO.pipe
w.write "123456789\r\n123456789\r\n"
w.close
pp io_pipe: r.gets("\r\n", 10)

__END__
$ ruby test.rb && RBENV_VERSION=truffleruby-24.2.1 ruby test.rb
========================================================================
Ruby Engine:   ruby 3.4.2
Using StringIO 3.1.7
{stringio: "123456789\r"}
{io_pipe: "123456789\r"}
========================================================================
Ruby Engine:   truffleruby 24.2.1
Using StringIO 3.0.1
{:stringio=>"123456789\r\n"}
{:io_pipe=>"123456789\r"}

I didn't make a TruffleRuby ticket for this. I'm guessing this is already a known issue, given that it silently failed to use a newer version of StringIO (even after I manually gem installed it). And it looks like it only affects the test which uses StringIO and probably doesn't affect real IO.