associate public entity fixture with category

fixes:

   2) Error:
 test_should_show_public_entity(PublicEntitiesControllerTest):
 ActionView::Template::Error: undefined method `name' for nil:NilClass
     /home/rsez/diafthoratelos.gr/ror/backkick/app/views/public_entities/show.html.erb:9:in
     `_app_views_public_entities_show_html_erb__112043201_87235160'