TestDialViaProxy: Fix bug in forward proxy

Closes #395

Confirmed library works correctly with a working forward proxy.

Author: nhooyr

Diff for: conn_test.go

@@ -535,6 +535,7 @@ func assertEcho(tb testing.TB, ctx context.Context, c *websocket.Conn) {
 	})
 
 	var act interface{}
+	c.SetReadLimit(1 << 30)
 	err := wsjson.Read(ctx, c, &act)
 	assert.Success(tb, err)
 	assert.Equal(tb, "read msg", exp, act)

Diff for: dial_test.go

@@ -361,21 +361,29 @@ func (fc *forwardProxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	}
 	w.Header().Set("PROXIED", "true")
 	w.WriteHeader(resp.StatusCode)
-	errc1 := xsync.Go(func() error {
-		_, err := io.Copy(w, resp.Body)
-		return err
-	})
-	var errc2 <-chan error
-	if bodyw, ok := resp.Body.(io.Writer); ok {
-		errc2 = xsync.Go(func() error {
-			_, err := io.Copy(bodyw, r.Body)
+	if resprw, ok := resp.Body.(io.ReadWriter); ok {
+		c, brw, err := w.(http.Hijacker).Hijack()
+		if err != nil {
+			http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
+			return
+		}
+		brw.Flush()
+
+		errc1 := xsync.Go(func() error {
+			_, err := io.Copy(c, resprw)
 			return err
 		})
-	}
-	select {
-	case <-errc1:
-	case <-errc2:
-	case <-r.Context().Done():
+		errc2 := xsync.Go(func() error {
+			_, err := io.Copy(resprw, c)
+			return err
+		})
+		select {
+		case <-errc1:
+		case <-errc2:
+		case <-r.Context().Done():
+		}
+	} else {
+		io.Copy(w, resp.Body)
 	}
 }