reverseproxy: content-length is wrong when the request doesnt fit in the request_buffer

[mcfedr]

how to do it

setup a reverse proxy with request_buffers

reverse_proxy example.com {
    request_buffers 1000
}

then send a request bigger than the buffer (i.e. > 1000 bytes in this case)

the content-length header to the backend will be set to 1000, but the body will actually be bigger, its io.MultiReader(buf, originalBody), - so the buffer plus the rest.

different backends have slightly different failure modes for this, im seeing this in my caddy logs

2023/09/20 19:38:08.374	ERROR	http.log.error	net/http: HTTP/1.x transport connection broken: http: ContentLength=1000 with Body length 64301	{"request": {"remote_ip": "127.0.0.1", "remote_port": "55244", "client_ip": "127.0.0.1", "proto": "HTTP/1.1", "method": "POST", "host": "127.0.0.1:8888", "uri": "/anything", "headers": {"User-Agent": ["curl/8.1.2"], "Accept": ["*/*"], "Accept-Encoding": ["deflate, gzip"], "Content-Type": ["text/plain"], "Content-Length": ["64301"]}}, "duration": 0.000745583, "status": 502, "err_id": "150qhej7z", "err_trace": "reverseproxy.statusError (reverseproxy.go:1248)"}

caused by this 2182270 which was fixing #5420

I would just offer a PR with a revert, but not sure how that affects the reason this commit was added