Fix: Omit hidden input before checkbox group

[ftes]

Omit hidden 'reset' input that appears before checkbox group if any checkboxes are checked.

What changed?

• Better preserve DOM order of form elements better when building form data.

Why make these changes?

A hidden input before a checkbox group can be used to reset the value if all checkboxes are unchecked.
However, if checkboxes are checked, then the hidden input should be effectively ignored.
Plug/Phoenix can handle this if the field values arrive in DOM order.

In forms like:

<input type="hidden" name="list" value="" />
<input type="checkbox" name="list[]" value="one" />
<input type="checkbox" name="list[]" value="two" />

PhoenixTest previously collected controls by selector category and stored FormData in a map. That lost DOM order and collapsed repeated field names, which could cause the hidden scalar entry to interfere with the checkbox array payload.

This PR is only a partial improvement. The ideal solution would be to submit form data in the exact DOM order a browser would, end to end.

[ftes]

This is a bit of a crux to handle multiple fields with same name.