support Namespaces when using var_export in toString()

[jclaveau]

Example:

            ['and',
                ['A_type', '=', 'INTERNE'],
                ['A_id', '>', 0],
                ['A_id', '>', 12],
                ['A_id', '>=', 100],
                ['A_id', '<', 2000],
                ['A_id', '=', 100],
                ['date', '=', DateTime::__set_state(array(
                   'date' => '2018-07-04 00:00:00.000000',
                   'timezone_type' => 3,
                   'timezone' => 'UTC',
                ))],
                ['E', '=', 'impression'],
            ]

In a given NS, DateTime while not work as meta-language part due to the lack of \ before DateTime