fix(net/ghttp): BufferWriter.Flush writes additional information afte…

…r custom response wrote (#4116)

net/ghttp/ghttp_z_unit_issue_test.go

@@ -656,3 +656,25 @@ func Test_Issue4108(t *testing.T) {
 		t.Assert(rsp.ReadAllString(), "hello")
 	})
 }
+
+// https://github.com/gogf/gf/issues/4115
+func Test_Issue4115(t *testing.T) {
+	s := g.Server(guid.S())
+	s.Use(func(r *ghttp.Request) {
+		r.Response.Writer.Write([]byte("hello"))
+	})
+	s.SetDumpRouterMap(false)
+	s.Start()
+	defer s.Shutdown()
+	time.Sleep(100 * time.Millisecond)
+
+	gtest.C(t, func(t *gtest.T) {
+		client := g.Client()
+		client.SetPrefix(fmt.Sprintf("http://127.0.0.1:%d", s.GetListenedPort()))
+
+		rsp, err := client.Get(ctx, "/")
+		t.AssertNil(err)
+		t.Assert(rsp.StatusCode, http.StatusOK)
+		t.Assert(rsp.ReadAllString(), "hello")
+	})
+}

net/ghttp/internal/response/response_buffer_writer.go

@@ -82,7 +82,7 @@ func (w *BufferWriter) Flush() {
 		w.Writer.WriteHeader(w.Status)
 	}
 	// Default status text output.
-	if w.Status != http.StatusOK && w.buffer.Len() == 0 {
+	if w.Status != http.StatusOK && w.buffer.Len() == 0 && w.Writer.BytesWritten() == 0 {
 		w.buffer.WriteString(http.StatusText(w.Status))
 	}
 	if w.buffer.Len() > 0 {