Fix Rack LintError from uppercase response headers for client_dev

[coalest]

The client dev server wasn't working, as apparently as of Rack 3 / HTTP 2 response headers should be lowercased.

Example stack trace

$ bundle exec rake client_dev
Puma starting in single mode...
* Puma version: 6.5.0 ("Sky's Version")
* Ruby version: ruby 3.3.3 (2024-06-12 revision f1c7b6f435) [x86_64-linux]
*  Min threads: 0
*  Max threads: 5
*  Environment: development
*          PID: 454193
* Listening on http://127.0.0.1:9292
* Listening on http://[::1]:9292
Use Ctrl-C to stop
Rack::Lint::LintError: uppercase character in header name: Content-Type (Rack::Lint::LintError)
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/rack-3.1.8/lib/rack/lint.rb:717:in `block in check_headers'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/rack-3.1.8/lib/rack/lint.rb:701:in `each'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/rack-3.1.8/lib/rack/lint.rb:701:in `check_headers'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/rack-3.1.8/lib/rack/lint.rb:76:in `response'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/rack-3.1.8/lib/rack/lint.rb:41:in `call'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/rack-3.1.8/lib/rack/show_exceptions.rb:31:in `call'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/rack-3.1.8/lib/rack/common_logger.rb:43:in `call'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/sinatra-4.1.1/lib/sinatra/base.rb:269:in `call'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/rack-3.1.8/lib/rack/content_length.rb:20:in `call'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/puma-6.5.0/lib/puma/configuration.rb:279:in `call'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/puma-6.5.0/lib/puma/request.rb:99:in `block in handle_request'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/puma-6.5.0/lib/puma/thread_pool.rb:389:in `with_force_shutdown'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/puma-6.5.0/lib/puma/request.rb:98:in `handle_request'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/puma-6.5.0/lib/puma/server.rb:468:in `process_client'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/puma-6.5.0/lib/puma/server.rb:249:in `block in run'
        /home/user/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/puma-6.5.0/lib/puma/thread_pool.rb:166:in `block in spawn_thread'

After these changes I finally got the client dev server working 🎉.

[tgxworld]

@coalest Thank you for the contribution. Can you add a changelog entry for this change in CHANGELOG.md please?

[coalest]

@tgxworld Added the changelog entry 👍

[nateberkopec]

Future TODO, we could add Rack::Lint to the test suite