net/http: omit Content-Length in Response.Write for 1xx or 204 status

net/http: omit Content-Length in Response.Write for 1xx or 204 status

Per RFC 7230, Section 3.3.2: "A server MUST NOT send a Content-Length
header field in any response with a status code of 1xx (Informational)
or 204 (No Content).".

Fixes #16942

Change-Id: I8006c76c126304e13618966e6eafb08a3885d3cd
Reviewed-on: https://go-review.googlesource.com/28351
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>