Fix JSON with unicode saving

Uses a patch to string repository https://github.com/joomla-framework/string/pull/6

A test case:
    $b = new Registry();
    $b->set('mykey', 'Тестовая строка в кириллице');
    echo $b->toString();

returns absolutely unreadable JSON with unicode sequences:
    {"mykey":"\u0422\u0435\u0441\u0442\u043e\u0432\u0430\u044f \u0441\u0442\u0440\u043e\u043a\u0430 \u0432 \u043a\u0438\u0440\u0438\u043b\u043b\u0438\u0446\u0435"}