Don't calculate Content-Length, let Rack do that for us; fixes #4440

Changes.md

@@ -2,6 +2,11 @@
 
 [Sidekiq Changes](https://github.com/mperham/sidekiq/blob/master/Changes.md) | [Sidekiq Pro Changes](https://github.com/mperham/sidekiq/blob/master/Pro-Changes.md) | [Sidekiq Enterprise Changes](https://github.com/mperham/sidekiq/blob/master/Ent-Changes.md)
 
+HEAD
+---------
+
+- Fix broken Web UI response when using NewRelic and Rack 2.1.2+. [#4440]
+
 6.0.4
 ---------
 

lib/sidekiq/web/application.rb

@@ -300,22 +300,20 @@ module Sidekiq
 
       resp = case resp
       when Array
+        # redirects go here
         resp
       else
+        # rendered content goes here
         headers = {
           "Content-Type" => "text/html",
           "Cache-Control" => "no-cache",
           "Content-Language" => action.locale,
           "Content-Security-Policy" => CSP_HEADER,
         }
-
+        # we'll let Rack calculate Content-Length for us.
         [200, headers, [resp]]
       end
 
-      resp[1] = resp[1].dup
-
-      resp[1][CONTENT_LENGTH] = resp[2].sum(&:bytesize).to_s
-
       resp
     end