fix: handle nested object fields in form data

[LeeWxx]

Fixes #14460

correcting object pointer handling in deep_set so nested form fields are properly stored in the same object hierarchy.

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[dummdidumm]

Thank you! Could you add a test to the sibling utils.spec.js (add another test to convert_formdata probably)?

[LeeWxx]

@dummdidumm Thanks! I’ve added a test for convert_formdata in utils.spec.js as suggested.

[dummdidumm]

thank you!