Prepopulating no longer working with latest 2.7.x Symphony branch

[pointybeard]

There was a change recently that meant prepopulating values in Select Box Link fields isn't working. You can see the issue at symphonycms/symphonycms#2751

The problem is SBL field is missing a canPrePopulate() method. It appears that it should have had one, but worked due to a missing check in the __viewNew method of contentPublish class.

The solution is to simply add the following to the SBL field (since the parent object returnsfalse):

    public function canPrePopulate() {
        return true;
    }

Doesn't appear to be any side effects to this addition since the field has always been able to prepopulate.

I will make a pull request for this change shortly.