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.
@ivanvanderbyl Any updates? Or a bug report so I can look into it?