Revert to direct asciicast JSON URL because Safari is the new IE

Safari fails XHR requests which redirect to other domain (some old CORS
bug they didn't fix for 5 years now)

app/serializers/asciicast_serializer.rb

@@ -13,7 +13,7 @@ class AsciicastSerializer < ActiveModel::Serializer
     if v0_url?
       object.stdout_frames_url
     else
-      asciicast_path(object, format: :json)
+      object.data_url
     end
   end
 

spec/serializers/asciicast_serializer_spec.rb

@@ -13,7 +13,7 @@ describe AsciicastSerializer do
     end
 
     it 'includes url' do
-      expect(subject['url']).to eq("/a/#{asciicast.to_param}.json")
+      expect(subject['url']).to eq(asciicast.file_url)
     end
 
     it 'includes snapshot' do