fix update() assigning values to columns marked with default

[nicoulaj]

This fixes a bug where update() does not exclude generated columns annotated with default attribute, leading to compilation errors when using #[derive(ormx::Table)] with postgres:

error returned from database: column "foo" can only be updated to DEFAULT

[NyxCode]

Hey, thanks for the PR.
#[ormx(default)] was intended to be used for columns with a default value.
For supporting generated columns, I suggest we introduce an other PG-specific attribute, maybe #[ormx(generated)], which would have the effect you implemented here for #[ormx(default)].

[nicoulaj]

Is it really PG specific though ? Mysql also has generated columns.

[NyxCode]

thats possible, never worked with it though.
But as I said, we need to differentiate between default and generated.

[NyxCode]

@nicoulaj Are you still willing to persue this? I think support for #[ormx(generated)] would be super cool.