fix 'split_unknown' arguments check

Describe changes

We updated assert_agg_scale_args arguments but didn't update the split_unknown function call, this fixes that.

It sounds like @StefanieAW was trying to use this in v2020.0.15 docker image, is it okay to install this fix into that image?

[x] Have you read the contributing guidelines for ihmeuw-demographics R packages?
[x] Have you successfully run devtools::check() locally?
[x] Have you updated or added function (and vignette if applicable) documentation? Did you update the 'man' and 'NAMESPACE' files with devtools::document()?
[x] Have you added in tests for the changes included in the PR?
[x] Do the changes follow the ihmeuw-demographics code style?
[ ] Do the changes need to be immediately included in a new build of docker-base or docker-internal? If so follow directions in those repositories to rebuild and redeploy the images.
[ ] Do the changes require updates to other repositories which use this package? If yes, make the necessary updates in those repos, and consider integration tests for those repositories.