FIX: Don’t try to serialize associations in PostRevisionSerializer

Currently, if an association is added as a tracked field in
`PostRevisor`, the `PostRevisionSerializer` class will try to serialize
it somehow. This will raise an error as ActiveRecord collection proxies
can't be serialized.

This patch addresses this issue by skipping any association tracked by
the `PostRevisor` class.

app/serializers/post_revision_serializer.rb

@@ -219,8 +219,13 @@ class PostRevisionSerializer < ApplicationSerializer
 
     # Retrieve any `tracked_topic_fields`
     PostRevisor.tracked_topic_fields.each_key do |field|
-      next if field == :tags
-      latest_modifications[field.to_s] = [topic.public_send(field)] if topic.respond_to?(field)
+      next unless topic.respond_to?(field)
+      topic
+        .public_send(field)
+        .then do |value|
+          next if value.try(:proxy_association)
+          latest_modifications[field.to_s] = [value]
+        end
     end
 
     latest_modifications["featured_link"] = [