Resolve observer attributes after booting has finished

- This ensure that all custom events have been registered by any traits first.
- It also prevents a new instance of the model from being created by the call to `observe` too early, since an instance created before booting has finished would be in an inconsistent state due to not all of the trait boot methods being run yet and not all of the initializer methods would be registered yet, so they would not be run either.

Author: willrowe

src/Illuminate/Database/Eloquent/Concerns/HasEvents.php

@@ -38,7 +38,9 @@ trait HasEvents
      */
     public static function bootHasEvents()
     {
-        static::observe(static::resolveObserveAttributes());
+        static::registerModelEvent('booted', function () {
+            static::observe(static::resolveObserveAttributes());
+        });
     }
 
     /**