Checkbox validation

[jnar]

Hi, thank you for this great work im currently using form_builder and saved me lots of time creating my forms.

I made a change because i was having issues with the checkbox validation when you have only one, around line 328 changed:
set_checkbox($id . '[]', $box->id, $selected)

for:
set_checkbox($id . ((count($boxes) > 1) ? '[]' : ''), $box->id, $selected)

Just to let you know.