Resolver had internal logic to prioritize locked versions when sorting
versions, however part of it was not being actually hit because of how
unlocking worked in the resolver: a package was allow to be unlocked
when that was explicit requested or when the list of unlocks was empty.
That did not make a lot of sense and other cases were working because
the explicit list of unlocks was getting "artificially filled".
Now we consider a package unlocked when explicitly requested (`bundle
update <package>`), or when everything is being unlocked (`bundle
install` with no lockfile or `bundle update`).
This makes things simpler and gets the edge case added as a test case
working as expected.
https://github.com/rubygems/rubygems/commit/b8e55087f0