Redirect a directory to its trailing slash only when something is served there

[sdogruyol]

Description of the Change

HTTP::StaticFileHandler#normalize_request_path redirects /dir → /dir/ whenever its @directory_listing flag is on. Kemal constructs the handler with the stdlib default (true) and never passes its own dir_listing setting down — it reads serve_static per request in directory_index instead. Result, with dir_listing and dir_index both off (the default):

GET /dir   → 302  Location: /dir/
GET /nope  → 404

Nothing is ever served at /dir/ in that configuration, so the redirect only tells the client that the directory is there. Directory enumeration under public/ for free.

The redirect now depends on serve_static: a directory URL gets its trailing slash when a listing or an index.html would answer at the slashed URL, and falls through to the same 404 as a missing path otherwise. The ≥ 1.17 branch overrides normalize_request_path; the legacy branch's is_dir check uses the same predicate. dir_index keeps the redirect on purpose — relative links in the index have to resolve against /dir/, not /.

Alternate Designs

• Pass directory_listing: into the constructor from Config#setup_static_file_handler. Simpler, but the flag would be frozen at boot while directory_index reads the config at request time; serve_static can be called after the handler exists (the specs do). Reading it in one place per request keeps the two decisions consistent.

Verification

Specs: neither on → /dir is 404 with no Location (fails on master: 302); listing on → 302 /dir/; index on → 302 /dir/. Full suite green (481), ameba and format clean.