fix: replace ember assign with Object.assign

[tsmalls93]

Ember assign has been deprecated in this RFC and @ember/polyfills is no longer available in Ember 5.x.

[aceol]

🙌🏻 Would be great to have this. Lots of warnings will popup with ember-data update (and it'll broke in v5).
If possible, change also this ligne in the readme:

    payload.data.attributes = assign(payload.data.attributes, attributes);