'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.'

def as_set(self, preserve_casing=False):

    if preserve_casing:
        return set(self._headers)
    return set(self._set)
