SNull<BelongsTo<T>> doesn't work, but Null<BelongsTo<T>> does

Just ran into this, writing it down so I remember to fix it later.

This works:

```
public var decidedBy:Null<BelongsTo<User>>;
```

This doesn't:

```
// Unsupported Record Type 
public var decidedBy:SNull<BelongsTo<User>>;  ufront.auth.model.User
```

Also this doesn't:

```
public var decidedBy:BelongsTo<Null<User>>;  // Null is not a value
```
