Support Option<Option<T>> for updating columns to be null

[tqwewe]

In some other libraries such as Diesel, it has support for Option<Option<T>> for an update struct.

This adds the following functionality:

• None - Don't update the column
• Some(None) - Update the column to be null
• Some(Some(value)) - Update the column to be value

Is this currently supported or planned to be supported?