Fix incorrect use of continue vs next

continue is not a Ruby keyword, but we've gotten lucky that only keys in the correct format are being inserted into redis. Fix this, and add a test that catches it.

If there is any chance of a mixed version environment, this version should be fully deployed before any version with https://gitlab.com/gitlab-org/sidekiq-reliable-fetch/-/merge_requests/21 otherwise working queue keys created with the new format will cause the older code to fail hard (undefined local variable or method continue).