@bogdan pointed out that a `loaded?` relation would not warn that the supplied
offset would be removed. This fixes that oversight.

https://github.com/rails/rails/commit/16a476e4f8f802774ae7c8dca2e59f4e672dc591#commitcomment-15706834

Although this second argument is probably not widely used, and would be
ignored anyway in the loaded? case, this could protect callers from gotchas.

[Ben Woosley & Victor Kmita]