[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
@gigr See: https://github.com/emberjs/data/blob/master/packages/activemodel-adapter/lib/system/active_model_serializer.js#L103-L132 the embedded records mixin was originally intended for active model serializers (gem / ember data serializer); so the `keyForRelationship` would select a suffix e.g. `id` or `_ids` and the `keyForAttribute` does not. These two methods are not defined in the JSONSerializer or RESTSerializer; however the ActiveModelSerializer does define them. 

So, when `includeRecords` is truthy the method chosen to get the key for the JSON payload is the `keyForAttribute` since there would be no need to make a choice based on a `belongsTo` or `hasMany` relationship kind. The `keyForAttribute` function only accepts an `attr` argument and the `keyForRelationship` method accepts both the attr and the kind of relationship as arguments.