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)
bring back constant to expose the enum mapping as HWIA.
Merge pull request #12783 from chancancode/eliminate_direct_json_gem_use

Eliminate `JSON.{parse,load,generate,dump}` and `def to_json`
Merge pull request #12788 from kylefritz/patch-1

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
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"
Merge pull request #12789 from claudiob/humanize-without-capitalizing

Add +capitalize+ option to Inflector.humanize
Extract method refactoring for Rails::Server#start