Module: fix multi-instance hook registrations

This fixes the problem that registering the same Hook multiple times
with the provideHook shortcut and no class name kept only one
implementation

library/Icinga/Application/Modules/Module.php

@@ -1289,7 +1289,7 @@ class Module
             $class = $implementation;
         }
 
-        Hook::register($name, $implementation, $class);
+        Hook::register($name, $class, $class);
         return $this;
     }