Remove a header from the set.  This raises an :exc:`KeyError` if the
        header is not in the set.

        .. versionchanged:: 0.5
            In older versions a :exc:`IndexError` was raised instead of a
            :exc:`KeyError` if the object was missing.

        :param header: the header to be removed.

def remove(self, header):
        """Remove a header from the set.  This raises an :exc:`KeyError` if the
        header is not in the set.

        .. versionchanged:: 0.5
            In older versions a :exc:`IndexError` was raised instead of a
            :exc:`KeyError` if the object was missing.

        :param header: the header to be removed.
        """
        key = header.lower()
        if key not in self._set:
            raise KeyError(header)
        self._set.remove(key)
        for idx, key in enumerate(self._headers):
            if key.lower() == header:
                del self._headers[idx]
                break
        if self.on_update is not None:
            self.on_update(self)