net/http/httputil: ignore CloseNotify when a non-background context is present

net/http/httputil: ignore CloseNotify when a non-background context is present

If the http.Request passed to ReverseProxy.ServeHTTP has a context
with a non-nil Done channel, don't watch the ResponseWriter's
CloseNotify channel.

Avoids starting an extra background goroutine in the common case.

Change-Id: I1328f3e02d3025caa0f446a2f20dfc14ef604c64
Reviewed-on: https://go-review.googlesource.com/c/go/+/376415
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Trust: Damien Neil <dneil@google.com>