caddy: Some proxied POST requests fail or hang when upstream does not consume body

1. What version of Caddy are you running (caddy -version)?

Caddy (untracked dev build)

2. What are you trying to do?

Upload files to a reverse proxy

3. What is your entire Caddyfile?

site.com {
  proxy / 127.0.0.1:1333 {
    max_fails 0
  }
}

4. How did you run Caddy (give the full command and describe the execution environment)?

$ sudo caddy -email="user@host.com" -agree

5. What did you expect to see?

Proxied requests and responses

6. What did you see instead (give full error messages and/or log)?

Requests were either not sent to the proxy server and responses were only partially sent (just the headers). Usually requests either end up hanging on the client side and never reach the proxy or the responses return the headers with a protocol exception (net::ERR_SPDY_PROTOCOL_ERROR)

This occurs when the proxy returns a status code outside the range of 200-399 despite having max_fails 0 set to prevent the backend from being marked as down. Even if this were the case, then this would not be expected behavior

No error messages are reported by Caddy when adding -log stdout to the command or anywhere else.

Code to reproduce (use the network inspector to view details)