Flush body when POST requests (#5128)

app/workers/activitypub/delivery_worker.rb

@@ -28,7 +28,7 @@ class ActivityPub::DeliveryWorker
   end
 
   def perform_request
-    @response = build_request.perform
+    @response = build_request.perform.flush
   end
 
   def response_successful?

app/workers/pubsubhubbub/delivery_worker.rb

@@ -37,7 +37,7 @@ class Pubsubhubbub::DeliveryWorker
   def callback_post_payload
     request = Request.new(:post, subscription.callback_url, body: payload)
     request.add_headers(headers)
-    request.perform
+    request.perform.flush
   end
 
   def blocked_domain?