If the user is using Server.SetKeepAlivesEnabled(false), assume they
have a reason and respect it. Make HTTP/2 behave like HTTP/1 (except a
bit nicer, since we have GOAWAY).

Updates golang/go#17717

Change-Id: I4a5ce324f0ac8653d83e75029063cd2e270a1048
Reviewed-on: https://go-review.googlesource.com/33153Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>