我在5.0.0.beta1 里解析模型都是nil 无法转换请问是怎么回事

`class ZYSwiftBaseModel: HandyJSON {
///code
var code: Int?
///msg
var msg: String = ""
///body
var body: T?

required init() {
}

func mapping(mapper: HelpingMapper) {

mapper <<<
code <-- ["code"]
mapper <<<
msg <-- ["msg"]
mapper <<<
body <-- ["body"]
}

func didFinishMapping() {

}
}`

` ///网络模型转换
guard let responseModel = ZYSwiftBaseModel.deserialize(from: response.responseString) else {
success?(T(), -502, "数据类型转换错误")
return
}`

Start by reproducing the ZYSwiftBaseModel.deserialize call using response.responseString and inspect how the shown mapping handles the response. Done means identifying why the result is nil and confirming that a valid response can be converted into a non-nil model.

Written by the indexing model from the issue text.