moodle-core-dialogue overwrites supplied configuration

As seen with the backup confirmcancel dialogue. If you first choose to stay, and then re-open it, the dialogue is display off screen.

This turns out to be because the configuration is passed by reference, and we then modify the values - e.g.:

    config.center =     config.centered && true;

    config.centered =   false;

This means that:

• the first time that the dialogue is displayed, it will be follow the value of config.centered;
• then config.centered is set to false;
• any subsequent time it is called, config.centered is now false, therefore config.center is also false