http: close after responding to HTTP/1.0 request without Connection: keep-alive

R=adg
CC=golang-dev
https://golang.org/cl/4245065

src/pkg/http/server.go

@@ -285,6 +285,9 @@ func (w *response) WriteHeader(code int) {
 		if !connectionHeaderSet {
 			w.SetHeader("Connection", "keep-alive")
 		}
+	} else if !w.req.ProtoAtLeast(1, 1) {
+		// Client did not ask to keep connection alive.
+		w.closeAfterReply = true
 	}
 
 	// Cannot use Content-Length with non-identity Transfer-Encoding.