Allow Volatile::extend() to work as expected

[dktapps]

Threaded objects are often poorly suited for runtime class extension because of their immutability characteristics. Classes which are not Threaded need to be adapted to extend Threaded (for example, overwriting a coerced array on a runtime-extended class crashes).

Because Volatile descends from Threaded, Volatile::extend() exists, but does not work as might be expected - it behaves exactly the same as Threaded::extend().

This PR changes Threaded::extend() to apply the scope it was called from, instead of always applying the Threaded parent.

A possible alternative to this could be to simply move extend() to Volatile completely, since as I said above Threaded isn't suited to code that doesn't know it's Threaded anymore due to immutability.

[sirsnyder]

To keep Volatile and Threaded consistent, I prefer the way of this PR.

[sirsnyder]

Thanks @dktapps !