Empty constructor for models with fields with default constructor

Empty constructor for `RdWrapper` is expected using this model
```
val RdContent = classdef {
property("myProperty", string)
field("myField", string).default("Default")
}

val RdWrapper = classdef {
field("content", RdContent)
}
```
but `RdContent` should be explicitly passed as argument: `RdWrapper(RdContent())`.

`RdWrapper` should have empty constructor because `myProperty` is property and `myField` has default value.