在mapping方法中做字段映射时 TransofrmOf初始化方法的回调不执行

```swift
class SearchNewsCellModel: HandyJSON {

var title:String? //标题
var content:String? //内容
var attrTitle: NSAttributedString?
var attrContent: NSAttributedString?

func mapping(mapper: HelpingMapper) {
mapper <<<
self.attrTitle <-- TransformOf(fromJSON: { (string) -> NSAttributedString? in
return self.title?.parseTitle()
}, toJSON: { (NSAttributedString) -> String? in
return nil
})

mapper <<<
self.attrContent <-- TransformOf(fromJSON: { (string) -> NSAttributedString? in
return self.content?.parseContent()
}, toJSON: { (NSAttributedString) -> String? in
return nil
})
}

}
```
fromJSON,toJSON这个两个闭包不执行， mapping函数能进入，版本是handyJSON1.8.0，Xcode9编译，swift版本3.2

The entry point is mapping(mapper:) with TransformOf in the supplied SearchNewsCellModel example; first reproduce it with HandyJSON 1.8.0 under Swift 3.2 and trace whether the callbacks are registered. Done means the fromJSON and toJSON closures execute during the relevant mapping operations.

Written by the indexing model from the issue text.