Improve update_expression method.

[bokuweb]

Is your feature request related to a problem? Please describe.

I think we should improve update_expression method like following.

• before

 let expression = User::update_expression()
            .set(User::status())
            .value(status);

• after

let expression = User::update_expression()
            .set_status(status);

Describe the solution you'd like

NA

Describe alternatives you've considered

NA

Additional context

NA