templates: bad content-length with httpError

[OpportunityLiu]

When a template returns an error with {{httpError xxx}}, the content-length header of the response will be set to a specific number, while the response body is empty.

Request to caddy docs:

curl -v https://caddyserver.com/docs/404
*   Trying 165.227.20.207:443...
* TCP_NODELAY set
* Connected to caddyserver.com (165.227.20.207) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: /etc/ssl/certs
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.3 (IN), TLS handshake, Server hello (2):
* TLSv1.3 (IN), TLS handshake, Encrypted Extensions (8):
* TLSv1.3 (IN), TLS handshake, Certificate (11):
* TLSv1.3 (IN), TLS handshake, CERT verify (15):
* TLSv1.3 (IN), TLS handshake, Finished (20):
* TLSv1.3 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.3 (OUT), TLS handshake, Finished (20):
* SSL connection using TLSv1.3 / TLS_AES_128_GCM_SHA256
* ALPN, server accepted to use h2
* Server certificate:
*  subject: CN=caddyserver.com
*  start date: Aug 31 02:32:51 2023 GMT
*  expire date: Nov 29 02:32:50 2023 GMT
*  subjectAltName: host "caddyserver.com" matched cert's "caddyserver.com"
*  issuer: C=US; O=Let's Encrypt; CN=R3
*  SSL certificate verify ok.
* Using HTTP2, server supports multi-use
* Connection state changed (HTTP/2 confirmed)
* Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
* Using Stream ID: 1 (easy handle 0x55f349347610)
> GET /docs/404 HTTP/2
> Host: caddyserver.com
> user-agent: curl/7.68.0
> accept: */*
> 
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
* Connection state changed (MAX_CONCURRENT_STREAMS == 250)!
* HTTP/2 stream 0 was not closed cleanly: PROTOCOL_ERROR (err 1)
* stopped the pause stream!
* Connection #0 to host caddyserver.com left intact
curl: (92) HTTP/2 stream 0 was not closed cleanly: PROTOCOL_ERROR (err 1)

Request to my server with following template

{{httpError 404}}

curl -v http://localhost/404.tpl.json
*   Trying 127.0.0.1:80...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 80 (#0)
> GET /404.tpl.json HTTP/1.1
> Host: localhost
> User-Agent: curl/7.68.0
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 404 Not Found
< Accept-Ranges: bytes
< Cache-Control: public,no-cache
< Content-Length: 18
< Content-Type: application/json
< Etag: "s2pib0i"
< Last-Modified: Wed, 18 Oct 2023 04:11:24 GMT
< Server: Caddy
< Date: Wed, 18 Oct 2023 04:11:26 GMT
< 
* transfer closed with 18 bytes remaining to read
* Closing connection 0
curl: (18) transfer closed with 18 bytes remaining to read

caddy version v2.7.5 h1:HoysvZkLcN2xJExEepaFHK92Qgs7xAiCFydN5x5Hs6Q=