Add convenience method to MaybeUndefined

[djc]

Description of the feature

It might be nice if MaybeUndefined knew how to update an Option<T> in place. Something like this:

fn update_if_defined(self, current: &mut Option<T>) {
    match self {
        MaybeUndefined::Value(new) => *current = Some(new),
        MaybeUndefined::Null => *current = None,
        MaybeUndefined::Undefined => {},
    };
}

What do you think? Worth thinking about whether this is a good name, too.

[sunli829]

This method is very useful. 🙂