artists: don't return domains in /artists/{id}.xml.

Don't return the `domains` field in /artists/{id}.{json,xml}. Fixes a
failure in /artists/{id}.xml:

    https://danbooru.donmai.us/artists/156646.xml

    <result success="false">
        undefined method `domains' for #<ArtistUrl:0x00005566dd340af0> Did you mean? DomainName
    </result>

`to_xml` passes down the `methods` param to all nested models, which
doesn't work.

app/controllers/artists_controller.rb

@@ -46,7 +46,7 @@ class ArtistsController < ApplicationController
   def show
     @artist = Artist.find(params[:id])
     @post_set = PostSets::Artist.new(@artist)
-    respond_with(@artist, methods: [:domains], include: [:urls])
+    respond_with(@artist)
   end
 
   def create

test/functional/artists_controller_test.rb

@@ -38,6 +38,13 @@ class ArtistsControllerTest < ActionDispatch::IntegrationTest
       end
     end
 
+    context "show action" do
+      should "work for xml responses" do
+        get artist_path(@masao.id), as: :xml
+        assert_response :success
+      end
+    end
+
     should "get the new page" do
       get_auth new_artist_path, @user
       assert_response :success