actually hook up the unknown capability fallback

Without this patch, `_fallback_capability` is set only on the UnknownCapability class, not on the Capability class.

To clarify, the very similar looking `register_capability` actually works because all subclasses share the dict that is attached to `registered_capability`, and no assignment to it happens, just operations *on* it. However, fallback happens via assignment, and hence we need this patch.