循环自定义$value和$index的时候出错
```
<div class="aui-slide-wrap">
    {{each user.img image index}}
    <div class="aui-slide-node bg-dark">
      <div style="background:url({{$image.img}});width:100%;height:100%;background-size:cover;background-position： center"></div>
    </div>
    {{/each}}
  </div>
```
user.img是数组

会报下面的错误：

```
TemplateError {
    "name": "RuntimeError",
    "path": "tpl-user",
    "message": "Cannot read property 'img' of undefined",
    "line": 6,
    "column": 22,
    "source": "{{$value.img}}"
}
```