Fix regression on editing private messages

v1.2.0beta9 has introduced a regression in edit of a private topic
(first post). Previously a check for no change in TopicsController was
made but it has been changed without considering that the topic could
be private.

By simply forcing a conversion of `topic.category_id` to integer, the case
where its value is nil is handled correctly as it was previously.

app/controllers/topics_controller.rb

@@ -133,7 +133,7 @@ class TopicsController < ApplicationController
     end
 
     changes.delete(:title) if topic.title == changes[:title]
-    changes.delete(:category_id) if topic.category_id == changes[:category_id].to_i
+    changes.delete(:category_id) if topic.category_id.to_i == changes[:category_id].to_i
 
     success = true
     if changes.length > 0