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, 9 insertions, 0 deletions

diff --git a/activesupport/test/core_ext/object/json_test.rb b/activesupport/test/core_ext/object/json_test.rb
new file mode 100644
index 0000000000..d3d31530df
--- /dev/null
+++ b/activesupport/test/core_ext/object/json_test.rb
@@ -0,0 +1,9 @@
+require 'abstract_unit'
+
+class JsonTest < ActiveSupport::TestCase
+  # See activesupport/test/json/encoding_test.rb for JSON encoding tests
+
+  def test_deprecated_require_to_json_rb
+    assert_deprecated { require 'active_support/core_ext/object/to_json' }
+  end
+end