Added a default to EventHandler.priority()

Fixed HandlerList.unregisterAll()
Fixed incorrect isAssignableFrom check in SimplePluginManager.getRegistrationClass()

By: zml2008 <zach@zachsthings.com>

paper-api/src/main/java/org/bukkit/event/EventHandler.java

@@ -11,5 +11,5 @@ public @interface EventHandler {
 
     Class<? extends Event> event();
 
-    EventPriority priority();
+    EventPriority priority() default EventPriority.NORMAL;
 }