compat: `/response-headers` endpoint missing headers

[sonbui00]

I got a different response.

Original
 curl https://httpbin.org/response-headers\?access_token\=faketoken\&token_type\=Bearer   
{
  "Content-Length": "128", 
  "Content-Type": "application/json", 
  "access_token": "faketoken", 
  "token_type": "Bearer"
}

go-httpbin
 curl http://httpbin:9100/response-headers\?access_token\=faketoken\&token_type\=Bearer  
{
  "access_token": [
    "faketoken"
  ],
  "token_type": [
    "Bearer"
  ]
}

[mccutchen]

I still think this is worth fixing, at least in part. Copying and lightly adapting a comment I left on the reporter's abandoned attempt to fix this issue:

---

If we're aiming for compatibility, it's worth exploring what the original httpbin's behavior is in weird circumstances? Like, what happens with a request like /response-headers?Content-Length=999999999? I think we need some test coverage for at least that edge case, to codify whatever behavior we decide on.

Turns out, the behavior is interesting (TLS-related noise elided below):

$ curl -v --http1.1 'https://httpbin.org/response-headers?Content-Type=text/plain&Content-Length=99999'
*   Trying 54.210.149.139:443...
* Connected to httpbin.org (54.210.149.139) port 443 (#0)
> GET /response-headers?Content-Type=text/plain&Content-Length=99999 HTTP/1.1
> Host: httpbin.org
> User-Agent: curl/8.1.2
> Accept: */*
> 
< HTTP/1.1 200 OK
< Date: Mon, 07 Aug 2023 22:40:24 GMT
< Content-Type: text/plain
< Content-Length: 99999
< Connection: keep-alive
< Server: gunicorn/19.9.0
< Access-Control-Allow-Origin: *
< Access-Control-Allow-Credentials: true
< 
{
  "Content-Length": [
    "122", 
    "99999"
  ], 
  "Content-Type": [
    "application/json", 
    "text/plain"
  ]
}
* transfer closed with 99877 bytes remaining to read
* Closing connection 0
curl: (18) transfer closed with 99877 bytes remaining to read

I can convince myself that letting the incoming request to this specific endpoint dictate the actual Content-Length of the response could be useful for testing weird behavior in HTTP client or proxy code, so I'd be in favor of matching the Python implementation there.

---

Note that there's a weird chicken/egg problem with the output from original httpbin above: It has the correct final Content-Length value as the first item under that key in the response JSON, but … how could it compute the accurate content length of the response without serializing the whole thing, including the final value for Content-Length?

I'm probably overthinking it or missing something comically obvious, but it turns out their implementation of this endpoint is a weird loop, presumably to solve this problem?