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*

Deprecate the delegation of Array bang methods in ActiveRecord::Delegation

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/test/cases/relation_test.rb

Make CollectionAssociation first/last with integer fetch with query

Conflicts:
	activerecord/CHANGELOG.md

[ci skip]

Let the SQLite rake tasks run without rails

Allow Relation#from to accept other relations with bind values.

Conflicts:
	activerecord/CHANGELOG.md

[ci skip]

do not break params filtering on nil values

Copy-edits

[ci skip]

[ci skip] Documentation on how to create custom rake tasks.

Remove unnecessary loop

"generates" applies to "collection radio" so it should be singular

I accidentally pushed this change to docrails before realizing that it's technically a code change, so I'm opening this pull request (and rolling back my change to docrails).

[ci skip]

form_for - fix :namespace and :as options clash

Conflicts:
	actionview/CHANGELOG.md

Removed semicolon and added space

[ci skip] Add missing setting locale code snippet.

This reverts commit ab5cd54b, reversing
changes made to cdc10c89.

Reason: This way of defining root path is still supported. See
https://github.com/rails/rails/blob/d262773ab7f0aae5de2d354ac2eca168e95b653d/actionpack/test/controller/routing_test.rb#L450-457

remove outdated docs

Fix FileStore#cleanup to no longer rely on missing each_key method

Enabled quirks mode on JSON.parse, fixes broken test in af9caae9

It turns out that ActionPack depends on the decoder to parse JSON
"fragments" (e.g. '"a string"', '1', 'null', etc), so we need to
enable quirks mode on JSON.parse. Also added coverage on the decoder
side to prevent regression.

[ci skip]

Cleanup: replace String concatenation by joining for excerpt helper

Update 4_0_release_notes.md

Active support --> Deprecations Code highlighted.

Added quotes when defining the precision for the `decimal` fields

Remove unused raw email fixtures

before:

```bash
$ rails generate migration AddDetailsToProducts price:decimal{5,2} supplier:references{polymorphic}
```

after:

```bash
$ rails generate migration AddDetailsToProducts 'price:decimal{5,2}' supplier:references{polymorphic}
```

The tests that used the raw_email_* fixtures were removed in d500ad3f

Replace JSON.load with JSON.parse

sense.  Let's load the object to ensure it's an array and count the
array.

At the point we want to add the reflection we already know the
reflection is of the AggregateReflection type so we can call a specific
method