Extract embedded `belongsTo` records properly

Fix a bug where embedded records two levels (or more) aren't properly materialised under some circumstances. The problem occurs when records are embedded within a `belongsTo` relationship.

The `sideload` method doesn't consider relationships/embedded records, by using
`extractRecordRepresentation` we'll recurse into the record (similarly
to `extractEmbeddedHasMany`).

Credits to @rykov for the test.