Minor code cleanup with group related actions (thanks brw12)

Originated from brw12 who noticed an incorrect variable name used in
an error message in actions/apigroupjoin.php:109

classes/User.php

@@ -616,12 +616,12 @@ class User extends Managed_DataObject
         return true;
     }
 
-    function isMember($group)
+    function isMember(User_group $group)
     {
         return $this->getProfile()->isMember($group);
     }
 
-    function isAdmin($group)
+    function isAdmin(User_group $group)
     {
         return $this->getProfile()->isAdmin($group);
     }