Embedded Records assert payload includes root element before extracting

This PR adds an assertion to `extractSingle` and `extractArray` within the `EmbeddedRecordsMixin` to help with debugging, especially when working with non-standard data which would first need to be normalized.

It also fixes a bug when extracting embedded belongsTo associations which use non-standard IDs.
I'll pull this line out as I'm working on a complete PR for this method. A lot of things don't work here.