ds-error validation is not reset for belongsTo relationship

### Environment

ember-cli: 3.12
ember: 3.12
ember-cp-validations: 4.0.0-beta.9

### Steps to Reproduce
When an attribute is validated against `ds-error` this validation is reset when the value of the attribute is changed. However, for a `belongsTo` relationship this is not true. Consider a situation when the app receives 2 errors from the backend. The user sees that, for example, a string and a dropdown value (which is bound to a relationship) are invalid. He alters the text field and the dropdown selected value, but only the text field becomes valid which is not consistent.
Here is a simple text for the described behaviour:
```ts
import { module, test } from 'qunit';
import { buildValidations, validator } from 'ember-cp-validations';
import Model, { attr, belongsTo } from '@ember-data/model';
import { setupTest } from 'ember-qunit';

module('Unit | Utility | ds-error issue', function(hooks) {
setupTest(hooks);

test('ds-error', async function(assert) {
const validations = buildValidations({
text: validator('ds-error'),
author: validator('ds-error'),
});
const userModel = Model.extend({
name: attr('string'),
});
const commentModel = Model.extend(validations, {
text: attr('string'),
author: belongsTo('test-user-model'),
});

this.owner.register('model:test-user-model', userModel);
this.owner.register('model:test-comment-model', commentModel);

const store = this.owner.lookup('service:store');
const user = store.createRecord('test-user-model', { id: 1, name: 'Charlie' });
const comment = store.createRecord('test-comment-model', { id: 1, text: 'note', author: user });

assert.ok(comment.validations.isValid, 'comment is valid');

comment.errors.add('text', 'text error');
comment.errors.add('author', 'author error');

assert.notOk(comment.validations.attrs.text.isValid, 'comment text is not valid');
assert.notOk(comment.validations.attrs.author.isValid, 'comment author is not valid');

comment.set('text', 'new text');
assert.ok(comment.validations.attrs.text.isValid, 'comment text is valid');

comment.set('author', store.createRecord('test-user-model', { id: 2, name: 'Bob' }));
// the below assertion is not satisfied
assert.ok(comment.validations.attrs.author.isValid, 'comment author valid');
});
});
```
So please tell me wether this is not a bug and how to reset the state of the validation to make the model valid.