Form Object RadioNodeList element fix

[laemtl]

HTMLFormElement.elements returns an HTMLFormControlsCollection, storing either RadioNodeLists or Elements.
This PR proposes a fix for the RadioNodeList case - it currently fails on theField.setAttribute (in updateForm).

To test, simply adds multiple inputs with the field name array syntax:

<label>Last name</label>
<input name="last_name[]">

<label>Last name</label>
<input name="last_name[]">

Thanks for this great library!

[bomsn]

Thank you.