Support ObservedBy on parent model classes

[adamthehutt]

When an Eloquent model extends another Eloquent model, the check for the ObservedBy attribute only considers the class itself. Any observers registered in the parent model are ignored.

This change adds support for the ObservedBy attribute in parent classes and traverses the hierarchy until the top Model class is reached.

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[taylorotwell]

Hey there! Could we get a test for this? Also should it keep going back up the tree in case there are multiple extensions?

[adamthehutt]

@taylorotwell You're too fast! :) I added another couple of commits after the initial draft pull request. These address the tree traversal issue you're asking about.

Will add a test ASAP.

[adamthehutt]

@taylorotwell Added a test. (Sorry about the sloppy commit history.)

[taylorotwell]

Thanks!

[patrickomeara]

This is a breaking change for me as I have a parent observer that creates polymorphic relationships, the grandchild model can't have these and doesn't have a morph map value.

Here is another example:

class MessageObserver
{
    public function created(Message $message) 
    {
        // send to all users
    }
}

#[ObservedBy(MessageObserver)]
class Message extends Model {}

class AdminMessage extends Message {}

Previously, creating an AdminMessage wouldn't send a notification to all users, now it is.

This PR takes away the granularity of adding the ObservedBy attribute where it's needed and assumes it can run for all child models.

Should we check for instanceOf in the observer methods?