docs: def update(self, iterable):
          """Add all the headers from the iterable to the set.
  
          :param iterable: updates the set with the items from the iterable.
          """
          inserted_any = False
          for header in iterable:
              key = header.lower()
              if key not in self._set:
                  self._headers.append(header)
                  self._set.add(key)
                  inserted_any = True
          if inserted_any and self.on_update is not None:
              self.on_update(self)