`json.to` fails with `ref Bar`

```nim
import json

type
  Foo = ref Bar
  Bar = object

discard "".parseJson.to Foo
```

> /usercode/in.nim(7, 21) template/generic instantiation of `to` from here
> /nim/lib/pure/json.nim(1217, 11) Error: type mismatch: got <typeof(nil)> but expected 'Bar = object'