docs: def as_set(self, preserve_casing=False):
          """Return the set as real python set type.  When calling this, all
          the items are converted to lowercase and the ordering is lost.
  
          :param preserve_casing: if set to `True` the items in the set returned
                                  will have the original case like in the
                                  :class:`HeaderSet`, otherwise they will
                                  be lowercase.
          """
          if preserve_casing:
              return set(self._headers)
          return set(self._set)