FIX: allow both meta_data and custom_fields when creating a topic

For some reasons, we have two ways of associating "custom fields" to a new topic:
using 'meta_data' and 'custom_fields'.

However, if we were to provide both arguments, the 'meta_data' would be overwritten
by any 'custom_fields' provided.

This commit ensures we can use both and merges the 'custom_fields' with the 'meta_data'.

spec/components/topic_creator_spec.rb

@@ -36,6 +36,18 @@ describe TopicCreator do
         expect(TopicCreator.create(moderator, Guardian.new(moderator), valid_attrs)).to be_valid
       end
 
+      it "supports both meta_data and custom_fields" do
+        opts = valid_attrs.merge(
+          meta_data: { import_topic_id: "foo" },
+          custom_fields: { import_id: "bar" }
+        )
+
+        topic = TopicCreator.create(admin, Guardian.new(admin), opts)
+
+        expect(topic.custom_fields["import_topic_id"]).to eq("foo")
+        expect(topic.custom_fields["import_id"]).to eq("bar")
+      end
+
       context 'regular user' do
         before { SiteSetting.min_trust_to_create_topic = TrustLevel[0] }