FIX: skip hidden posts while generating canonical url.

Previously, while generating the topic page's canoncial url we used the current post number. It will create invalid canonical path if the topic has whsiper posts. Now we only taking the visible posts for current page index calculation.

lib/topic_view.rb

@@ -122,7 +122,8 @@ class TopicView
       if @page > 1
         "?page=#{@page}"
       else
-        page = ((@post_number - 1) / @limit) + 1
+        posts_count = unfiltered_posts.where("post_number <= ?", @post_number).count
+        page = ((posts_count - 1) / @limit) + 1
         page > 1 ? "?page=#{page}" : ""
       end