FIX: Prevent ActorHandler from attempting to apply updates to categories or tags (#208)

Reference: https://meta.discourse.org/t/activitypub-plugin/266794/432?u=angus

Author: angusmcleod

lib/discourse_activity_pub/actor_handler.rb

@@ -40,6 +40,7 @@ def build_actor
     end
 
     def update_or_create_user
+      return if model_type.present? && model_type != "User"
       @model_type = "User"
       return invalid_opts unless valid_actor?
 

spec/lib/discourse_activity_pub/actor_handler_spec.rb

@@ -32,6 +32,15 @@
       end
     end
 
+    context "when the actor is for a category or tag" do
+      let!(:actor) { Fabricate(:discourse_activity_pub_actor_group) }
+
+      it "does nothing" do
+        result = described_class.update_or_create_user(actor)
+        expect(result).to eq(nil)
+      end
+    end
+
     context "when actor has user" do
       let!(:user) { Fabricate(:user) }