Allow disabling of AutoTimestamp on insertion event if values are not null

[codeconsole]

Useful if importing data. If set to false, AutoTimestamp will only set create and update fields if they are null during insertion events. Defaults to existing behavior which is true.

Setting

grails:
    gorm:
        events:
            insertOverwrite: false

allows:

class User {
    Date dateCreated
    Date lastUpdated
}

def user = new User()
user.dateCreated = new Date() - 1
user.save(flush:true)
assert user.lastUpdated != null
assert user.dateCreated != user.lastUpdated

[jdaugherty]

For some reason, I thought it didn't set the created if it was already set. I'm ok with this change, but this needs test coverage. We shouldn't be changing this without associated tests.

[codeconsole]

@jdaugherty yeah, it's normal behavior is unexpected and that is why I introduced this in the first place.

I added test coverage.

[jdaugherty]

We need to add documentation for these features in gorm.grails.org. Can you create a ticket and do this before we go live with Grails 7?