Fix deadlock that can occur when child live thread tries to load a constant after writing to the stream.

It's tough for people without the knowledge of where the `get` and
friends integration test helpers are defined to find documentation
for them. Add a link to the main integration test documentation.

* Give the section a header to distinguish it from the general doc.
* Replace backticks with + signs to fit SDoc.
* Use double quoted strings.
* Clarify how `parsed_body` works — it doesn't depend on `as` anymore.

They are not part of the public API

When initializing an `ActionDispatch::Http::Headers` object it takes a request object (Rails 5) whereas before it took a hash (Rails 4.x) but the documented example still shows a hash given to the constructor (due to commit 34fa6658) so this is just a documentation change to use the new `from_hash` method introduced in that earlier commit.

Thinking .. relative to files is not natural, we are used
to think "parent of a directory", and we have __dir__
nowadays.

In c546a2b0 parameter handling in AC test cases was changed to
round tripping through encoders/decoders so that they matched
reality and in 0adb8f8f the old methods were removed but the
`html_format?` method was overlooked.

At GitHub we need to handle parameter encodings that are not UTF-8. This
patch allows us to specify encodings per parameter per action.

Hash syntax auto-correcting breaks alignments. 411ccbda

A few have been left for aesthetic reasons, but have made a pass
and removed most of them.

Note that if the method `foo` returns an array, `foo << 1`
is a regular push, nothing to do with assignments, so
no self required.

In some test runs `ActionController::Base.enable_fragment_cache_logging` would be false,
based on the test order.

Turns out it was off because we assigned the config to the `@controller` variable, and
not on `ActionController::Base`.

The test failure was reproducible with, and now passes after this:

```
bin/test test/controller/log_subscriber_test.rb --seed 19918
```

The entry was a result of a combination of changes in Action View
and Action Controller.

Implement naive partial caching mechanism.

Add test for LogSubscriber

Use ActionView::Base#log_payload to store log_subscriber's payload, so we can pass cache result into it.

Fixed tests

Remove useless settings

Check if #log_payload exists before calling it. Because other classes also includes CacheHelper but don't have is attribute

Use @log_payload_for_partial_reder instead of #log_payload to carry ActionView's payload.

Update test's hash syntax

Add configuration to enable/disable fragment caching logging

Remove unless test and add new test to ensure cache info won't effect next rendering's log

Move :enable_fragment_cache_logging config from ActionView to ActionPack

Apply new config to tests

Update actionview's changelog

Update configuration guide

Improve actionview's changelog

Refactor PartialRenderer#render and log tests

Mute subscriber's log instead of disabling instrumentation.

Fix typo, remove useless comment and use new hash syntax

Improve actionpack's log_subscriber test

Fix rebase mistake

Apply new config to all caching intstrument actions

In integration test when specify the "Accept" header with "xhr: true"
option, the Accept header is overridden with a default xhr Accept
header. The issue only affects HTTP header "Accept" but not CGI variable
"HTTP_ACCEPT".

For example:

    get '/page', headers: { 'Accept' => 'application/json' }, xhr: true

    # This is WRONG! And the response.content_type is also affected.
    # It should be "application/json"
    assert_equal "text/javascript, text/html, ...", request.accept
    assert_equal 'text/html', response.content_type

The issue is in `ActionDispatch::Integration::RequestHelpers`. When
setting "xhr: true" the helper sets a default HTTP_ACCEPT if blank.
But the code doesn't consider supporting both HTTP header style and
CGI variable style.

For detail see this GitHub issue:
https://github.com/rails/rails/issues/25859

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

When a `GET` request is sent `as: :json` in an integration test the test
should use Rack's method override to change to a post request so the
paramters are included in the postdata. Otherwise it will not encode the
parameters correctly for the integration test.

Because integration test sets up it's own middleware,
`Rack::MethodOverride` needs to be included in the integration tests as
well.

`headers ||= {}` was moved so that headers are never nil. They should
default to a hash.

Fixes #26033

[Eileen M. Uchitelle & Aaron Patterson]

Screwed up both the left and right hand sides!

The tests were written with the common false value seen in Rails apps,
show that intent in the code.

Should also fix the build on 5-0-stable.

Looked odd, so completely detached from the other necessary part of
the implementation.

The coder that Psych passes in has a `tag` method we can use to detect
which serialization format we're reviving for. Use it and make it clearer
alongside the `load_tags` fiddling.

If we were to serialize an `ActionController::Parameters` on Psych 2.0.8, we'd get:

```yaml
--- !ruby/hash:ActionController::Parameters
key: :value
```

Because 2.0.8 didn't store instance variables, while 2.0.9 did:
https://github.com/tenderlove/psych/commit/8f84ad0fc711a82a1040def861cb121e8985fd4c

That, coupled with 2.0.8 calling `new` instead of `allocate` meant parameters was
deserialized just fine:
https://github.com/tenderlove/psych/commit/af308f8307899cb9e1c0fffea4bce3110a1c3926

However, if users have 2.0.8 serialized parameters, then upgrade to Psych 2.0.9+ and
Rails 5, it would start to blow up because `initialize` will never be called, and thus
`@parameters` will never be assigned. Hello, `NoMethodErrors` on `NilClass`! :)

To fix this we register another variant of the previous serialization format and take
it into account in `init_with`.

I've tested this in our app and previously raising code now deserializes like a champ.
I'm unsure how to test this in our suite because we use Psych 2.0.8 and don't know how
to make us use 2.0.9+ for just one test.

By changing ActionController::Parameter's superclass, Rails 5 also changed
the YAML serialization format.

Since YAML doesn't know how to handle parameters it would fallback to its
routine for the superclass, which in Rails 4.2 was Hash while just Object
in Rails 5. As evident in the tags YAML would spit out:

4.2: !ruby/hash-with-ivars:ActionController::Parameters
5.0: !ruby/object:ActionController::Parameters

Thus when loading parameters YAML from 4.2 in Rails 5, it would parse a
hash dump as it would an Object class.

To fix this we have to provide our own `init_with` to be aware of the past
format as well as the new one. Then we add a `load_tags` mapping, such that
when the YAML parser sees `!ruby/hash-with-ivars:ActionController::Parameters`,
it knows to call our `init_with` function and not try to instantiate it as
a normal hash subclass.

Before this change, posted parameters would leak across requests. The included
test case failed like so:

      1) Failure:
    TestCaseTest#test_multiple_mixed_method_process_should_scrub_rack_input:
    --- expected
    +++ actual
    @@ -1 +1 @@
    -{"bar"=>"an bar", "controller"=>"test_case_test/test", "action"=>"test_params"}
    +{"foo"=>"an foo", "bar"=>"an bar", "controller"=>"test_case_test/test", "action"=>"test_params"}

An argument could be made that this situation isn't encountered often and that
one should limit the number of requests per test case, but I still think the
parameter leaking is an unexpected side-effect.