自定义属性映射时获取不到json字符串的值

这个是我的属性 `var paramValues = MoLastUpdateTime()`

下面是我的自定义映射方法：
```
func mapping(mapper: HelpingMapper) {
mapper <<<
self.paramValues <-- TransformOf(fromJSON: { (string) -> MoLastUpdateTime? in
// 我在这里获取到的string值为nil,请问是不是我哪里写错了？（补充：我的json数据里面是有这个字段的）
}, toJSON: { (model) -> String? in

})
}
```

Start with the custom TransformOf mapping for paramValues and trace the JSON deserialization path to determine why the fromJSON closure receives nil even though the field is present. Reproduce the mapping with a JSON value for that field; done means the closure receives the expected string and the property is populated.

Written by the indexing model from the issue text.