Function toFormData

toFormData(obj, hasBrackets?, hasIndex?): FormData
对象转 FormData 格式
Parameters
- obj: any
  源数据
- hasBrackets: boolean = false
  是否带括号
- hasIndex: boolean = false
  是否带 index
Returns FormData
Example
```
toFormData({a: 1, b: 2}); /// FormData
```