Performs a join using the union join function.

def merge(left, right, how='inner', key=None, left_key=None, right_key=None,
          left_as='left', right_as='right'):
    """ Performs a join using the union join function. """
    return join(left, right, how, key, left_key, right_key,
                join_fn=make_union_join(left_as, right_as))