middleware upgrade (#4861)

* middleware upgrade

* restore @entries init

lib/sidekiq/middleware/chain.rb

@@ -90,12 +90,12 @@ module Sidekiq
       end
 
       def add(klass, *args)
-        remove(klass) if exists?(klass)
+        remove(klass)
         entries << Entry.new(klass, *args)
       end
 
       def prepend(klass, *args)
-        remove(klass) if exists?(klass)
+        remove(klass)
         entries.insert(0, Entry.new(klass, *args))
       end
 
@@ -132,7 +132,7 @@ module Sidekiq
       def invoke(*args)
         return yield if empty?
 
-        chain = retrieve.dup
+        chain = retrieve
         traverse_chain = proc do
           if chain.empty?
             yield
@@ -144,6 +144,8 @@ module Sidekiq
       end
     end
 
+    private
+
     class Entry
       attr_reader :klass