Support SaveAs - Enum write as number format

[Lightczx]

Description

Currently we can parse Enum from number but can't write back as number. One possible way is define a new model class and make it as a middleware. But it's a little annoying converting types from time to time. Another way is using the description attribute,and it's also a littlt tricky. I think we can make the enums parse from and write to as number by default, and only make it write string when there is Format specified or DescriptionAttribute present.