new default decayimage on category icon change

[evoltech]

When a new category icon is added a default decayimage is added, but when a category icon is updated a new decayimage is not added.

[vanc1337]

This was an issue of colliding keys in the database, specifically category_id, to solve it I added the lines:

  $cat_id=$category->id;
  ORM::factory('decayimage')->where(array('category_id' => $cat_id))->delete_all();

just before the creation of the decay image database row circa line ~250 in the function decayimage_ushahidi_filter_category_save. This should delete any existing entry for decay image category with the same category id. Testing has proven successful for me, provided this does not cause any unforeseen issues this bug can probably be closed.