[BUGFIX] Event restriction is not applied to Demand subclasses

I haven't used the IsEvent restriction in combination with an extended
demand class yet, but as a developer extending eventnews I expect the
IsEvent restriction to also be effective on subclasses of the eventnews
Demand class.

Classes/Hooks/AbstractDemandedRepository.php

@@ -24,7 +24,7 @@ class AbstractDemandedRepository
      */
     public function modify(array $params)
     {
-        if (get_class($params['demand']) !== Demand::class) {
+        if (!($params['demand'] instanceof Demand)) {
             return;
         }