Incorrect `Content-Type` in the API response from linting.org

[EtonDoze]

Expected: application/json
Actual: text/html

This even confuses Postman leading to render incorrectly.

Reproducible steps

~> curl -v https://rules.linting.org/rulesets/info/
*   Trying 172.67.136.246:443...
* Connected to rules.linting.org (172.67.136.246) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*  CAfile: /etc/ssl/cert.pem
*  CApath: none
* (304) (OUT), TLS handshake, Client hello (1):
* (304) (IN), TLS handshake, Server hello (2):
* (304) (OUT), TLS handshake, Client hello (1):
* (304) (IN), TLS handshake, Server hello (2):
* (304) (IN), TLS handshake, Unknown (8):
* (304) (IN), TLS handshake, Certificate (11):
* (304) (IN), TLS handshake, CERT verify (15):
* (304) (IN), TLS handshake, Finished (20):
* (304) (OUT), TLS handshake, Finished (20):
* SSL connection using TLSv1.3 / AEAD-AES256-GCM-SHA384
* ALPN, server did not agree to a protocol
* Server certificate:
*  subject: C=US; ST=California; L=San Francisco; O=Cloudflare, Inc.; CN=sni.cloudflaressl.com
*  start date: Aug 14 00:00:00 2022 GMT
*  expire date: Oct 15 07:37:18 2022 GMT
*  subjectAltName: host "rules.linting.org" matched cert's "*.linting.org"
*  issuer: C=US; ST=California; L=San Jose; O=PayPal, Inc.; OU=Crypto Mgt Corp EJBCA; CN=PayPal ZIA Issuing CA
*  SSL certificate verify ok.
> GET /rulesets/info/ HTTP/1.1
> Host: rules.linting.org
> User-Agent: curl/7.79.1
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Date: Wed, 17 Aug 2022 23:23:09 GMT
< Content-Type: text/html; charset=utf-8
< Transfer-Encoding: chunked
< Connection: keep-alive
< last-modified: Mon, 01 Aug 2022 16:06:16 GMT
< access-control-allow-origin: *
< expires: Wed, 17 Aug 2022 23:29:42 GMT
< Cache-Control: max-age=600
< x-proxy-cache: MISS
< x-github-request-id: D3C0:62A8:B6AC3:CF2EA:62FD778E
< via: 1.1 varnish
< age: 0
< x-served-by: cache-lax10620-LGB
< x-cache: HIT
< x-cache-hits: 1
< x-timer: S1660778590.546976,VS0,VE97
< vary: Accept-Encoding
< x-fastly-request-id: 8924dfde4901754e2a22d63bcee72e7d3ccc5397
< CF-Cache-Status: DYNAMIC
< Expect-CT: max-age=604800, report-uri="https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct"
< Report-To: {"endpoints":[{"url":"https:\/\/a.nel.cloudflare.com\/report\/v3?s=lurDO%2FlodRQGlob4WJj9RSPCsU4dP2uPTsT5zBnJTjcarDxaix5sr2cNkcvA0AOrqrgwM30VzZI%2FZHAFNZ7y6W42GMyBi0EI7By5yjOzBySgJDmZtr0LOjkFyqMWYJyXVa0sEg%3D%3D"}],"group":"cf-nel","max_age":604800}
< NEL: {"success_fraction":0,"report_to":"cf-nel","max_age":604800}
< Server: cloudflare
< CF-RAY: 73c627e86b627c50-LAX
< alt-svc: h3=":443"; ma=86400, h3-29=":443"; ma=86400
< 
{
"rules": {