Decorator to add header with XML version declaration to output from FN.

def add_xml_declaration(fn):
    """
    Decorator to add header with XML version declaration to output from FN.
    """
    @wraps(fn)
    def add_xml_declaration_decorator(*args, **kwargs):
        return '<?xml version="1.0" encoding="UTF-8"?>\n\n' + fn(
            *args,
            **kwargs
        )

    return add_xml_declaration_decorator