[Fix] embed relations serialization

[florianJacques]

Fixed the serialization of embed relations.
These relationships were not necessarily loaded, and could not use their attribute mutators.

Checklist

• Add tests and ensure they pass

[florianJacques]

Hello,
Why Coding Standards / phpcs (pull_request)
Fail ?

[GromNaN]

In order to review the PR properly, could you add some tests that are fixed by the change?

Why Coding Standards / phpcs (pull_request) Fail?

GitHub Action fails to commit the fixes that are found by phpcs.

----------------------------------------------------------------------
FILE                                                  FIXED  REMAINING
----------------------------------------------------------------------
src/Eloquent/EmbedsRelations.php                      2      0
src/Eloquent/DocumentModel.php                        1      0
----------------------------------------------------------------------
A TOTAL OF 3 ERRORS WERE FIXED IN 2 FILES
----------------------------------------------------------------------

[florianJacques]

Yes of course, I will write the tests in the next few days
For CI errors, can I do something?

[GromNaN]

For CI errors, can I do something?

You can run vendor/bin/phpcbf locally to fix the CS issues.

[florianJacques]

Thanks, Done! I'll write the tests when I have some time...

[florianJacques]

Adding test ok