Performance optimization for AttributeSet#deep_dup

Skip the call to #dup, since it does a shallow copy of attributes,
which is wasted effort, since #deep_dup then replaces that
shallow copy with a #deep_dup of the given attributes.

This change addresses slowness in ActiveRecord initialization
introduced starting in Rails 5.0.

activerecord/lib/active_record/attribute_set.rb

@@ -64,9 +64,7 @@ module ActiveRecord
     end
 
     def deep_dup
-      dup.tap do |copy|
-        copy.instance_variable_set(:@attributes, attributes.deep_dup)
-      end
+      self.class.new(attributes.deep_dup)
     end
 
     def initialize_dup(_)