Avoid double time zone converter decoration when user-defined timestamp attribute with implicit type

If type is given as a symbol, it will lookup a fresh type object via
`Type.lookup`, but if type is omitted, `type_for_attribute` will lookup
the database type which is already decorated by `define_attribute`.

To avoid the double decoration, skip extra decoration if its type is
already decorated.

activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb

@@ -6,6 +6,10 @@ module ActiveRecord
   module AttributeMethods
     module TimeZoneConversion
       class TimeZoneConverter < DelegateClass(Type::Value) # :nodoc:
+        def self.new(subtype)
+          self === subtype ? subtype : super
+        end
+
         def deserialize(value)
           convert_time_to_time_zone(super)
         end

activerecord/lib/active_record/locking/optimistic.rb

@@ -178,6 +178,10 @@ module ActiveRecord
     # `nil` values to `lock_version`, and not result in `ActiveRecord::StaleObjectError`
     # during update record.
     class LockingType < DelegateClass(Type::Value) # :nodoc:
+      def self.new(subtype)
+        self === subtype ? subtype : super
+      end
+
       def deserialize(value)
         super.to_i
       end

activerecord/test/cases/attributes_test.rb

@@ -74,10 +74,12 @@ module ActiveRecord
       end
 
       starts_at_type = klass.type_for_attribute(:starts_at)
+
       assert_equal 3, starts_at_type.precision
       assert_equal 2, starts_at_type.limit
       assert_equal 1, starts_at_type.scale
 
+      assert_kind_of Type::DateTime, starts_at_type
       assert_instance_of Time, klass.new.starts_at
     end
 
@@ -85,9 +87,18 @@ module ActiveRecord
       with_timezone_config aware_attributes: true, zone: "Pacific Time (US & Canada)" do
         klass = Class.new(OverloadedType) do
           attribute :starts_at, :datetime, precision: 3, default: -> { Time.now.utc }
+          attribute :ends_at, default: -> { Time.now.utc }
         end
 
+        starts_at_type = klass.type_for_attribute(:starts_at)
+        ends_at_type   = klass.type_for_attribute(:ends_at)
+
+        assert_instance_of AttributeMethods::TimeZoneConversion::TimeZoneConverter, starts_at_type
+        assert_instance_of AttributeMethods::TimeZoneConversion::TimeZoneConverter, ends_at_type
+        assert_kind_of Type::DateTime, starts_at_type.__getobj__
+        assert_kind_of Type::DateTime, ends_at_type.__getobj__
         assert_instance_of ActiveSupport::TimeWithZone, klass.new.starts_at
+        assert_instance_of ActiveSupport::TimeWithZone, klass.new.ends_at
       end
     end
 

activerecord/test/schema/schema.rb

@@ -1127,6 +1127,7 @@ ActiveRecord::Schema.define do
     t.string :overloaded_string_with_limit, limit: 255
     t.string :string_with_default, default: "the original default"
     t.string :inferred_string, limit: 255
+    t.datetime :starts_at, :ends_at
   end
 
   create_table :users, force: true do |t|