Upstream 404 is not always passed through as 404 status

[martinemde]

Reported in slack:

% curl -I https://gem.coop/api/v1/gems/nonexistentpackage12345.json
HTTP/2 200 
content-type: text/plain
server: gem.coop
x-served-by: cache-ams2100103-AMS
date: Mon, 13 Oct 2025 11:32:01 GMT

Expected 404 status code but returns text in the body "404" with successful response.

Compare with rubygems.org:

% curl -I https://rubygems.org/api/v1/gems/nonexistentpackage12345.json
HTTP/2 404 
content-type: text/plain; charset=utf-8
x-frame-options: SAMEORIGIN
x-xss-protection: 0
x-content-type-options: nosniff
x-permitted-cross-domain-policies: none
referrer-policy: strict-origin-when-cross-origin
cross-origin-opener-policy: same-origin