docs: def add_item(self, item_url, item_metadata):
          """ Add the given item to the cache database, updating
          the existing metadata if the item is already present
  
          :type item_url: String or Item
          :param item_url: the URL of the item, or an Item object
          :type item_metadata: String
          :param item_metadata: the item's metadata, as a JSON string
  
  
          """
          c = self.conn.cursor()
          c.execute("DELETE FROM items WHERE url=?", (str(item_url),))
          self.conn.commit()
          c.execute("INSERT INTO items VALUES (?, ?, ?)",
                    (str(item_url), item_metadata, self.__now_iso_8601()))
          self.conn.commit()
          c.close()