Fixed a compatibility issue with the `Oj` gem

`Time#as_json`, `Date#as_json` and `DateTime#as_json` incorrectly depends on a
delegation that is set up in `active_support/json/encoding`. We cannot simply
require that file in `core_ext/object/json` because it would cause a circular
dependency problem (see #12203 for background). We should instead rely on AS's
autoload to load that file for us on-demand.

To trigger autoload correctly, we need to reference the `AS::JSON::Encoding`
constant instead of using the delegated version.

Fixes #16131.

activesupport/CHANGELOG.md

+*   Fixed a compatibility issue with the `Oj` gem when cherry-picking the file
+    `active_support/core_ext/object/json` without requiring `active_support/json`.
+
+    Fixes #16131.
+
+    *Godfrey Chan*
+
 *   Make `Hash#with_indifferent_access` copy the default proc too.
 
     *arthurnn*, *Xanders*

activesupport/lib/active_support/core_ext/object/json.rb

@@ -162,7 +162,7 @@ def as_json(options = nil) #:nodoc:
 
 class Time
   def as_json(options = nil) #:nodoc:
-    if ActiveSupport.use_standard_json_time_format
+    if ActiveSupport::JSON::Encoding.use_standard_json_time_format
       xmlschema(ActiveSupport::JSON::Encoding.time_precision)
     else
       %(#{strftime("%Y/%m/%d %H:%M:%S")} #{formatted_offset(false)})
@@ -172,7 +172,7 @@ def as_json(options = nil) #:nodoc:
 
 class Date
   def as_json(options = nil) #:nodoc:
-    if ActiveSupport.use_standard_json_time_format
+    if ActiveSupport::JSON::Encoding.use_standard_json_time_format
       strftime("%Y-%m-%d")
     else
       strftime("%Y/%m/%d")
@@ -182,7 +182,7 @@ def as_json(options = nil) #:nodoc:
 
 class DateTime
   def as_json(options = nil) #:nodoc:
-    if ActiveSupport.use_standard_json_time_format
+    if ActiveSupport::JSON::Encoding.use_standard_json_time_format
       xmlschema(ActiveSupport::JSON::Encoding.time_precision)
     else
       strftime('%Y/%m/%d %H:%M:%S %z')