Fix `Rack::Handler::HTTP` spec hanging issue

[postmodern]

For some reason the Rack::Handler::HTTP specs hang with a long timeout and eventually this error:

[Tue Mar  4 17:25:44 2025] /data/home/postmodern/code/net-http-server/spec/rack/handler/helpers/test_request.rb:12:in 'block in TestRequest#call'
/data/home/postmodern/code/net-http-server/spec/rack/handler/helpers/test_request.rb:10:in 'Array#each'
/data/home/postmodern/code/net-http-server/spec/rack/handler/helpers/test_request.rb:10:in 'TestRequest#call'
/data/home/postmodern/code/net-http-server/vendor/bundle/ruby/3.4.0/gems/rack-2.2.12/lib/rack/static.rb:161:in 'Rack::Static#call'
/data/home/postmodern/code/net-http-server/vendor/bundle/ruby/3.4.0/gems/rack-2.2.12/lib/rack/lint.rb:50:in 'Rack::Lint#_call'
/data/home/postmodern/code/net-http-server/vendor/bundle/ruby/3.4.0/gems/rack-2.2.12/lib/rack/lint.rb:38:in 'Rack::Lint#call'
/data/home/postmodern/code/net-http-server/lib/rack/handler/http.rb:138:in 'Rack::Handler::HTTP#call'
/data/home/postmodern/code/net-http-server/lib/net/http/server/daemon.rb:122:in 'Net::HTTP::Server::Daemon#serve'
/data/home/postmodern/code/net-http-server/vendor/bundle/ruby/3.4.0/gems/gserver-0.0.1/lib/gserver.rb:268:in 'block (2 levels) in GServer#start'