[Bugfix] embedded records mixin: key variable needed to be declared

- `key` was not declared prior to assignment in `serializeBelongsTo` method
- when embedded records using `serializeBelongsTo` use `keyForAttribute` not `keyForRelationship` like `serializeHasMany` does
Why does the belongs-to side call `getKeyForAttribute` but not the has-many side above on line 147?