Hugo `0.55.0` introduced some new interface types for `Page` etc.
This worked great in general, but there were cases where this would fail in `where` and `sort`.
One such example would be sorting by `MenuItem.Page.Date` where `Page` on `MenuItem` was a small subset of the bigger `page.Page` interface.
This commit fixes that by unwrapping such interface values.
Fixes#5989
Bjørn Erik Pedersen
8d898ad667