Moved all JSON core extensions into core_ext/object/json

TL;DR The primary driver is to remove autoload surprise.

This is related to #12106. (The root cause for that ticket is that
json/add defines Regexp#to_json among others, but here I'll reproduce
the problem without json/add.)

Before:

   >> require 'active_support/core_ext/to_json'
   => true
   >> //.as_json
   NoMethodError: undefined method `as_json' for //:Regexp
     from (irb):3
     from /Users/godfrey/.rvm/rubies/ruby-2.0.0-p195/bin/irb:16:in `<main>'
   >> //.to_json
   => "\"(?-mix:)\""
   >> //.as_json
   => "(?-mix:)"

After:

   >> require 'active_support/core_ext/to_json'
   => true
   >> //.as_json
   => "(?-mix:)"

This is because ActiveSupport::JSON is autoloaded the first time
Object#to_json is called, which causes additional core extentions
(previously defined in active_support/json/encoding.rb) to be loaded.

When someone require 'active_support/core_ext', the expectation is
that it would add certain methods to the core classes NOW. The
previous behaviour causes additional methods to be loaded the first
time you call `to_json`, which could cause nasty surprises and other
unplesant side-effects.

This change moves all core extensions in to core_ext/json. AS::JSON is
still autoloaded on first #to_json call, but since it nolonger
include the core extensions, it should address the aforementioned bug.

*Requiring core_ext/object/to_json now causes a deprecation warnning*

1 files changed, 2 insertions, 4 deletions

diff --git a/guides/source/active_support_core_extensions.md b/guides/source/active_support_core_extensions.md
index e6b849e4c9..af29594e54 100644
--- a/guides/source/active_support_core_extensions.md
+++ b/guides/source/active_support_core_extensions.md
@@ -420,11 +420,9 @@ NOTE: Defined in `active_support/core_ext/object/with_options.rb`.
 
 ### JSON support
 
-Active Support provides a better implementation of `to_json` than the +json+ gem ordinarily provides for Ruby objects. This is because some classes, like +Hash+ and +OrderedHash+ needs special handling in order to provide a proper JSON representation.
+Active Support provides a better implementation of `to_json` than the +json+ gem ordinarily provides for Ruby objects. This is because some classes, like +Hash+, +OrderedHash+ and +Process::Status+ needs special handling in order to provide a proper JSON representation.
 
-Active Support also provides an implementation of `as_json` for the <tt>Process::Status</tt> class.
-
-NOTE: Defined in `active_support/core_ext/object/to_json.rb`.
+NOTE: Defined in `active_support/core_ext/object/json.rb`.
 
 ### Instance Variables