Optimize uri normalization (#4212)

* Add dependency on idn-ruby to speed up URI normalization

* Use normalized_host instead of normalize.host when applicable

When we are only interested in the normalized host, calling normalized_host
avoids normalizing the other components of the URI as well as creating a
new object

app/workers/pubsubhubbub/delivery_worker.rb

@@ -43,7 +43,7 @@ class Pubsubhubbub::DeliveryWorker
   end
 
   def host
-    Addressable::URI.parse(subscription.callback_url).normalize.host
+    Addressable::URI.parse(subscription.callback_url).normalized_host
   end
 
   def headers