Make #deep_dup use #allocate instead of #new

This change preserves the speedup made in a24912cb1d (by avoiding
the wasted shallow dup of @attributes) while ensuring that the
performance of #deep_dup won't be tied to the performance of #initialize

activerecord/lib/active_record/attribute_set.rb

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