API fix for beets script collection

@xhochy kindly added some new API support for the beets script collection to the beets web service and built support into the resolver. I opted to change the API in a slightly different way than originally proposed. This small change makes the resolver compatible with my version of the change.

Change to beets is [here](https://github.com/sampsyo/beets/commit/70b528ed817783436e6331ff7f347fcd941fc29f).