FIX: update categories topic_count when converting topic to a PM and vice versa

spec/models/topic_converter_spec.rb

@@ -5,7 +5,7 @@ describe TopicConverter do
   context 'convert_to_public_topic' do
     let(:admin) { Fabricate(:admin) }
     let(:author) { Fabricate(:user) }
-    let(:category) { Fabricate(:category) }
+    let(:category) { Fabricate(:category, topic_count: 1) }
     let(:private_message) { Fabricate(:private_message_topic, user: author) } # creates a topic without a first post
     let(:first_post) { Fabricate(:post, topic: private_message, user: author) }
     let(:other_user) { private_message.topic_allowed_users.find { |u| u.user != author }.user }
@@ -19,6 +19,7 @@ describe TopicConverter do
         expect(topic).to be_valid
         expect(topic.archetype).to eq("regular")
         expect(topic.category_id).to eq(SiteSetting.uncategorized_category_id)
+        expect(topic.category.topic_count).to eq(1)
       end
 
       describe 'when uncategorized category is not allowed' do
@@ -38,6 +39,7 @@ describe TopicConverter do
             .where(read_restricted: false).order('id asc').first
 
           expect(topic.category_id).to eq(first_category.id)
+          expect(topic.category.topic_count).to eq(2)
         end
       end
 
@@ -46,6 +48,7 @@ describe TopicConverter do
           topic = TopicConverter.new(private_message, admin).convert_to_public_topic(category.id)
 
           expect(topic.reload.category).to eq(category)
+          expect(topic.category.topic_count).to eq(2)
         end
       end
 
@@ -86,13 +89,16 @@ describe TopicConverter do
   context 'convert_to_private_message' do
     let(:admin) { Fabricate(:admin) }
     let(:author) { Fabricate(:user) }
-    let(:topic) { Fabricate(:topic, user: author) }
+    let(:category) { Fabricate(:category) }
+    let(:topic) { Fabricate(:topic, user: author, category_id: category.id) }
 
     context 'success' do
       it "converts regular topic to private message" do
         private_message = topic.convert_to_private_message(admin)
         expect(private_message).to be_valid
         expect(topic.archetype).to eq("private_message")
+        expect(topic.category_id).to eq(nil)
+        expect(category.topic_count).to eq(0)
       end
 
       it "updates user stats" do