martian(h2): fix half-close propagation downstream

Use http2.Response.Close() to send RST_STREAM frame.
Reimplement writeFlusher as h2Writer.

This is to be tested in Sauce Connect due to lack of testing infrastructure for http2 at the moment.
I added #869 to fix that.

internal/martian/proxy_handler.go

@@ -236,7 +236,7 @@ func (p proxyHandler) tunnel(name string, rw http.ResponseWriter, req *http.Requ
 
 		cc = []copier{
 			{"outbound " + name, crw, req.Body},
-			{"inbound " + name, writeFlusher{rw, rc}, crw},
+			{"inbound " + name, makeH2Writer(rw, rc, req), crw},
 		}
 	default:
 		err := fmt.Errorf("unsupported protocol version: %d", req.ProtoMajor)
@@ -337,27 +337,36 @@ func (p proxyHandler) handleRequest(rw http.ResponseWriter, req *http.Request) {
 	}
 }
 
-type writeFlusher struct {
-	rw io.Writer
-	rc *http.ResponseController
+type h2Writer struct {
+	w     io.Writer
+	flush func() error
+	close func() error
 }
 
-func (w writeFlusher) Write(p []byte) (n int, err error) {
-	n, err = w.rw.Write(p)
+func makeH2Writer(rw http.ResponseWriter, rc *http.ResponseController, req *http.Request) *h2Writer {
+	return &h2Writer{
+		w:     rw,
+		flush: rc.Flush,
+		close: req.Body.Close,
+	}
+}
+
+func (w h2Writer) Write(p []byte) (n int, err error) {
+	n, err = w.w.Write(p)
 
 	if n > 0 {
-		if err := w.rc.Flush(); err != nil {
+		if err := w.flush(); err != nil {
 			log.Errorf(context.TODO(), "got error while flushing response back to client: %v", err)
 		}
 	}
 
 	return
 }
 
-func (w writeFlusher) CloseWrite() error {
-	// This is a nop implementation of closeWriter.
-	// It avoids printing the error log "cannot close write side of inbound CONNECT tunnel".
-	return nil
+func (w h2Writer) CloseWrite() error {
+	// Close request body to signal the end of the request.
+	// This results RST_STREAM frame with error code NO_ERROR to be sent to the other side.
+	return w.close()
 }
 
 func (p proxyHandler) writeErrorResponse(rw http.ResponseWriter, req *http.Request, err error) {