Fix Worker for use with DATABASE_URL

Calling QC.default_conn_adapter when no connection exists will use `QC_DATABASE_URL` or `DATABASE_URL` to establish one or fail with a meaningful exception if the variables are not set.
I might be missing something, but without any assertion here, what is that testing and in which case would it fail?

(sorry if that's stupid, I have a cold and feel like shit but want to get that bug fix out with QC 3.0.1 release...)