diff --git a/lib/sidekiq/core_ext.rb b/lib/sidekiq/core_ext.rb
index d862843e..e69de29b 100644
--- a/lib/sidekiq/core_ext.rb
+++ b/lib/sidekiq/core_ext.rb
@@ -1,75 +0,0 @@
-# frozen_string_literal: true
-begin
-  require 'active_support/core_ext/class/attribute'
-rescue LoadError
-  # A dumbed down version of ActiveSupport 5.1.0's
-  # Class#class_attribute helper.
-  class Module
-    # Removes the named method, if it exists.
-    def remove_possible_method(method)
-      if method_defined?(method) || private_method_defined?(method)
-        undef_method(method)
-      end
-    end
-
-    # Removes the named singleton method, if it exists.
-    def remove_possible_singleton_method(method)
-      singleton_class.instance_eval do
-        remove_possible_method(method)
-      end
-    end
-  end
-
-  class Class
-    def class_attribute(*attrs)
-      instance_reader = true
-      instance_writer = true
-
-      attrs.each do |name|
-        remove_possible_singleton_method(name)
-        define_singleton_method(name) { nil }
-
-        ivar = "@#{name}"
-
-        remove_possible_singleton_method("#{name}=")
-        define_singleton_method("#{name}=") do |val|
-          singleton_class.class_eval do
-            remove_possible_method(name)
-            define_method(name) { val }
-          end
-
-          if singleton_class?
-            class_eval do
-              remove_possible_method(name)
-              define_method(name) do
-                if instance_variable_defined? ivar
-                  instance_variable_get ivar
-                else
-                  singleton_class.send name
-                end
-              end
-            end
-          end
-          val
-        end
-
-        if instance_reader
-          remove_possible_method name
-          define_method(name) do
-            if instance_variable_defined?(ivar)
-              instance_variable_get ivar
-            else
-              self.class.public_send name
-            end
-          end
-        end
-
-        if instance_writer
-          remove_possible_method "#{name}="
-          attr_writer name
-        end
-      end
-    end
-
-  end
-end
diff --git a/lib/sidekiq/job_retry.rb b/lib/sidekiq/job_retry.rb
index aa5069ba..b8ce81bd 100644
--- a/lib/sidekiq/job_retry.rb
+++ b/lib/sidekiq/job_retry.rb
@@ -203,7 +203,7 @@ module Sidekiq
     end
 
     def delay_for(worker, count, exception)
-      worker && worker.sidekiq_retry_in_block? && retry_in(worker, count, exception) || seconds_to_delay(count)
+      worker && worker.sidekiq_retry_in_block && retry_in(worker, count, exception) || seconds_to_delay(count)
     end
 
     # delayed_job uses the same basic formula
diff --git a/lib/sidekiq/worker.rb b/lib/sidekiq/worker.rb
index 9eccd634..c85ddcb6 100644
--- a/lib/sidekiq/worker.rb
+++ b/lib/sidekiq/worker.rb
@@ -4,7 +4,6 @@ require 'sidekiq/core_ext'
 
 module Sidekiq
 
-
   ##
   # Include this module in your worker class and you can easily create
   # asynchronous jobs:
@@ -149,6 +148,62 @@ module Sidekiq
         Sidekiq::Client.new(pool).push(item)
       end
 
+      def class_attribute(*attrs)
+        instance_reader = true
+        instance_writer = true
+
+        attrs.each do |name|
+          singleton_class.instance_eval do
+            undef_method(name) if method_defined?(name) || private_method_defined?(name)
+          end
+          define_singleton_method(name) { nil }
+
+          ivar = "@#{name}"
+
+          singleton_class.instance_eval do
+            m = "#{name}="
+            undef_method(m) if method_defined?(m) || private_method_defined?(m)
+          end
+          define_singleton_method("#{name}=") do |val|
+            singleton_class.class_eval do
+              undef_method(name) if method_defined?(name) || private_method_defined?(name)
+              define_method(name) { val }
+            end
+
+            if singleton_class?
+              class_eval do
+                undef_method(name) if method_defined?(name) || private_method_defined?(name)
+                define_method(name) do
+                  if instance_variable_defined? ivar
+                    instance_variable_get ivar
+                  else
+                    singleton_class.send name
+                  end
+                end
+              end
+            end
+            val
+          end
+
+          if instance_reader
+            undef_method(name) if method_defined?(name) || private_method_defined?(name)
+            define_method(name) do
+              if instance_variable_defined?(ivar)
+                instance_variable_get ivar
+              else
+                self.class.public_send name
+              end
+            end
+          end
+
+          if instance_writer
+            m = "#{name}="
+            undef_method(m) if method_defined?(m) || private_method_defined?(m)
+            attr_writer name
+          end
+        end
+      end
+
     end
   end
 end
diff --git a/test/test_retry_exhausted.rb b/test/test_retry_exhausted.rb
index 0a542b5f..fdf39e94 100644
--- a/test/test_retry_exhausted.rb
+++ b/test/test_retry_exhausted.rb
@@ -64,7 +64,7 @@ class TestRetryExhausted < Sidekiq::Test
         # successful
       end
 
-      refute NewWorker.exhausted_called?
+      refute NewWorker.exhausted_called
     end
 
     it 'does not run exhausted block when job successful on last retry' do
@@ -72,7 +72,7 @@ class TestRetryExhausted < Sidekiq::Test
         # successful
       end
 
-      refute NewWorker.exhausted_called?
+      refute NewWorker.exhausted_called
     end
 
     it 'does not run exhausted block when retries not exhausted yet' do
@@ -82,7 +82,7 @@ class TestRetryExhausted < Sidekiq::Test
         end
       end
 
-      refute NewWorker.exhausted_called?
+      refute NewWorker.exhausted_called
     end
 
     it 'runs exhausted block when retries exhausted' do
@@ -92,7 +92,7 @@ class TestRetryExhausted < Sidekiq::Test
         end
       end
 
-      assert NewWorker.exhausted_called?
+      assert NewWorker.exhausted_called
     end
 
 
@@ -104,7 +104,7 @@ class TestRetryExhausted < Sidekiq::Test
       end
       raised_error = raised_error.cause
 
-      assert new_worker.exhausted_called?
+      assert new_worker.exhausted_called
       assert_equal raised_error.message, new_worker.exhausted_job['error_message']
       assert_equal raised_error, new_worker.exhausted_exception
     end
@@ -117,7 +117,7 @@ class TestRetryExhausted < Sidekiq::Test
       end
       raised_error = raised_error.cause
 
-      assert old_worker.exhausted_called?
+      assert old_worker.exhausted_called
       assert_equal raised_error.message, old_worker.exhausted_job['error_message']
       assert_nil new_worker.exhausted_exception
     end