多层级映射问题

```
extension CGRect: HandyJSON {
public mutating func mapping(mapper: HelpingMapper) {
mapper <<<
self.origin.x <-- ["x"]
mapper <<<
self.origin.y <-- ["y"]
mapper <<<
self.size.width <-- ["width"]
mapper <<<
self.size.height <-- ["height"]
}

public mutating func didFinishMapping() {
// print(self)
}
}
```

我这样写不能赋值，有什么办法吗

The issue provides only a CGRect mapping snippet and no repository file or test. Reproduce the nested-property assignment with that snippet, then inspect HandyJSON's mapper implementation and existing mapping tests; done means x, y, width, and height deserialize into CGRect successfully.

Written by the indexing model from the issue text.