Panic when primary key of active model is not set during update

[billy1624]

When a primary key is not included in an active model, sea-orm panics. Could this be handled in a better way?
https://github.com/SeaQL/sea-orm/blob/master/src/query/update.rs#L95

let active_model = cake::ActiveModel {
    // Don't include ID ...
    ..Default::default()
};

let products = Cake::update(active_model).exec(&db).await?;

@acidic9 thanks for raising this issue

[tyt2y3]

merge to #96