Revert "Merge pull request rails#46444 from eileencodes/revert-forgetting-assignment-changes"

This reverts commit 586436d, reversing
changes made to 866e053.

This is an unrevert of rails#46282. I
should have reverted the revert sooner but it kept falling off my radar.

Closes rails#46446

Author: eileencodes

activemodel/lib/active_model/attribute.rb

@@ -175,6 +175,19 @@ def type_cast(value)
           type.deserialize(value)
         end
 
+        def forgetting_assignment
+          # If this attribute was not persisted (with a `value_for_database`
+          # that might differ from `value_before_type_cast`) and `value` has not
+          # changed in place, we can simply dup this attribute to avoid
+          # deserialize / cast / serialize calls from computing the new
+          # attribute's `value_before_type_cast`.
+          if !defined?(@value_for_database) && !changed_in_place?
+            dup
+          else
+            super
+          end
+        end
+
         private
           def _original_value_for_database
             value_before_type_cast