In #12203, the JSON core extensions were moved into the `core_ext`
folder. Unfortunately, there are some corresponding requires that
were left behind. The problem is partially addressed in #12710, this
commit fixes the rest.

Do not expose internal state in the public encoder API (i.e. as_json)

See [1] for why this is not a good idea.

As part of this refactor, circular reference protection in as_json has
been removed and the corresponding error class has been deprecated.

As discussed with @jeremy, circular reference error is considered
programmer errors and protecting against it is out of scope for
the encoder.

This is again based on the excellent work by @sergiocampama in #11728.

[1]: https://github.com/intridea/multi_json/pull/138#issuecomment-24468223

This saved about 46 array allocations per request on an extremely simple
application.  The delegation happened in the notification subsystem
which is a hotspot, so this should result in even more savings with
larger apps.

Squashed commit of the following:

commit 41eef0d1479526f7de25fd4391d98e61c126d9f5
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Nov 6 16:32:31 2013 -0800

    speed up notifications

commit 586b4a18656f66fb2c518fb8e8fee66a016e8ae6
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Nov 6 16:31:05 2013 -0800

    speed up runtime registry methods

commit b67d074cb4314df9a88438f785868cef77e583d7
Author: Aaron Patterson <aaron.patterson@gmail.com>
Date:   Wed Nov 6 16:28:12 2013 -0800

    change method name and make it public

constant.

`view_assigns` can use the precalculated sets and remove instance
variables without allocating any extra arrays

Add +capitalize+ option to Inflector.humanize

So strings can be humanized without being capitalized:

    'employee_salary'.humanize                    # => "Employee salary"
    'employee_salary'.humanize(capitalize: false) # => "employee salary"

rdoc: favicon source shouldn't begin with a slash to reference asset pipeline resource [ci skip]
Conflicts:
	actionview/lib/action_view/helpers/asset_tag_helper.rb

Eliminate `JSON.{parse,load,generate,dump}` and `def to_json`

JSON.{dump,generate} offered by the JSON gem is not compatiable with
Rails at the moment and can cause a lot of subtle bugs when passed
certain data structures. This changed all direct usage of the JSON gem
in internal Rails code to always go through AS::JSON.{decode,encode}.

We also shouldn't be implementing `to_json` most of the time, and
these occurances are replaced with an equivilent `as_json`
implementation to avoid problems down the road.

See [1] for all the juicy details.

[1]: intridea/multi_json#138 (comment)

Fixed Object#as_json and Struct#as_json with options

These methods now takes the same options as Hash#as_json, for example:

    struct = Struct.new(:foo, :bar).new
    struct.foo = "hello"
    struct.bar = "world"
    json = struct.as_json(only: [:foo]) # => {foo: "hello"}

This is extracted from PR #11728 from @sergiocampama, see also the
discussion in #11460.

This allows to assign both `String` and `Symbol` values to the enum
without having to call `to_sym`, which is a security problem.

Clarify HTML Safe Translations

I think it's confusing to say "Use them in views without escaping." We
use all keys in views without escaping - the escaping is done for us
automatically _unless_ we call html_safe or the key ends in _html.

Renamed generated_feature_methods to generated_association_methods.

Conflicts:
	activerecord/lib/active_record/associations/builder/association.rb
	activerecord/lib/active_record/associations/builder/singular_association.rb
	activerecord/test/cases/base_test.rb

Use 1.9 Hash syntax consistently

Refactor File.expand_path usage to remove additional File.join