Avoid calling attr_protected if _accessible is used

When attr_accessible is used, we may assume that watcher_ids and
watcher_user_ids is already protected from mass assignment. Therefore we do not
need to perform additional actions.

Also, when attr_protected and attr_accessible is used on the same model, AR
throws nasty "Declare either attr_protected or attr_accessible for *, but not
both." errors.

vendor/plugins/acts_as_watchable/lib/acts_as_watchable.rb

@@ -20,7 +20,7 @@ module Redmine
               { :include => :watchers,
                 :conditions => ["#{Watcher.table_name}.user_id = ?", user_id] }
             }
-            attr_protected :watcher_ids, :watcher_user_ids
+            attr_protected :watcher_ids, :watcher_user_ids if accessible_attributes.nil?
           end
         end
       end