Fix 'deleteable' option on 'single_upload'.

The value of the delete input field "$childPost['delete']", which is inserted on the array containing the configurations at the preSubmit() method, was not being verified at the onSubmit() method.
This was causing the file to "always" (if no new file was set for uploading on that field) be deleted when submitting the form just by having the 'deleteable' option active.