Traversing fieldset elements fix

I was trying to get plupload to display on the "Add datastream" form but it would not show up - when I looked through the add datastream form, I saw the form had a fieldset with the managed_file form element inside. Once I looked at the islandora_plupload module, I saw the manner in which it processed fieldset elements seemed incorrect. The pull request simplifies the code so it doesn't try to see if the fieldset has children; it just goes through it as if it is a subform (which is how the initial 'islandora_plupload_alter_managed_files' expects it.