code: def setdefault(self, key: str, value: str) -> str:
          
          set_key = key.lower().encode("latin-1")
          set_value = value.encode("latin-1")
  
          for idx, (item_key, item_value) in enumerate(self._list):
              if item_key == set_key:
                  return item_value.decode("latin-1")
          self._list.append((set_key, set_value))
          return value
docs: If the header `key` does not exist, then set it to `value`.
          Returns the header value.