[BUG]

**Describe the bug**
if base Controller have a type of struct or []struct{} ,it have errs like this :

code :

This issue only occurs in POST requests, there are no issues with Get

I don't know why?

others all code can see https://github.com/langzishen/iris_demo

**To Reproduce**
Steps to reproduce the behavior:
1. [...]

**Expected behavior**
A clear and concise description of what you expected to happen.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Desktop (please complete the following information):**
- OS: [e.g. ubuntu, windows]

**iris.Version**
- e.g. v12.2.0 or master

Please make sure the bug is reproducible over the `master` branch:

```sh
$ cd PROJECT
$ go get -u github.com/kataras/iris/v12@master
$ go run .
```

**Additional context**
Add any other context about the problem here.