net/http: Document Content-Length behavior for servers

[jsha]

What version of Go are you using (go version)?

Go 1.9

Does this issue reproduce with the latest release?

Yes.

For a Go HTTP server, we noticed that sometimes it was setting the Content-Length header on responses, and sometimes it was not. The user code for the HTTP server didn't set Content-Length anywhere, so it must be the stdlib code. After some digging in the stdlib code, we found that Content-Length will be automatically set on small response bodies only. It would be good to document this in the net/http godocs.

[mvdan]

/cc @bradfitz @tombergan

[gopherbot]

Change https://golang.org/cl/115040 mentions this issue: net/http: document Server's implicit Content-Length response header