FIX: admins could never remove self from messages

app/models/topic.rb

@@ -698,6 +698,9 @@ SQL
       topic_user = topic_allowed_users.find_by(user_id: user.id)
       if topic_user
         topic_user.destroy
+        # we can not remove ourselves cause then we will end up adding
+        # ourselves in add_small_action
+        removed_by = Discourse.system_user if user.id == removed_by.id
         add_small_action(removed_by, "removed_user", user.username)
         return true
       end

spec/controllers/topics_controller_spec.rb

@@ -1134,9 +1134,21 @@ describe TopicsController do
         xhr :put, :make_banner, topic_id: topic.id
         expect(response).to be_success
       end
-
     end
+  end
 
+  describe 'remove_allowed_user' do
+    it 'admin can be removed from a pm' do
+
+      admin = log_in :admin
+      user = Fabricate(:user)
+      pm = create_post(user: user, archetype: 'private_message', target_usernames: [user.username, admin.username])
+
+      xhr :put, :remove_allowed_user, topic_id: pm.topic_id, username: admin.username
+
+      expect(response.status).to eq(200)
+      expect(TopicAllowedUser.where(topic_id: pm.topic_id, user_id: admin.id).first).to eq(nil)
+    end
   end
 
   describe 'remove_banner' do