具体堆栈如下：

ErrorMsg：reflect: reflect.Value.Set using value obtained using unexported field
Stack：1. reflect: reflect.Value.Set using value obtained using unexported field
1). github.com/chanxuehong/wechat/mp/core.(Server).ServeHTTP
/root/workspace/ees-server-go_games_xNfU/vendor/github.com/chanxuehong/wechat/mp/core/server.go:434
2). banban/app/service/wechatmp.(wechatmp).Callback
/root/workspace/ees-server-go_games_xNfU/app/service/wechatmp/mp.go:62
3). banban/app/api.(wechatmp).Callback
/root/workspace/ees-server-go_games_xNfU/app/api/wechatmp.go:15
4). banban/app/service.(middlewareService).Error
/root/workspace/ees-server-go_games_xNfU/app/service/middleware.go:232
5). banban/app/service.(*middlewareService).Ctx
/root/workspace/ees-server-go_games_xNfU/app/service/middleware.go:213