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.

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]

Fixes #25926

Previously we'd only assign a response parser when a request came through
Action Dispatch integration tests. This made calls to `parsed_body` when a TestResponse
was manually instantiated — though own doing or perhaps from a framework — unintentionally
blow up because no parser was set at that time.

The response can lookup a parser entirely through its own ivars. Extract request encoder to
its own file and assume that a viable content type is present at TestResponse instantiation.

Since the default response parser is a no-op, making `parsed_body` equal to `body`, no
exceptions will be thrown.

Felt that += overwriting the path variable was a little too hidden.

Make the outcomes easier to spot with an if-else branch.

When running tests with `--enable-frozen-string-literal` or
`# frozen_string_literal: true`, it's currently attempted to mutate the path
string in order to append the format, causing a `RuntimeError`.

```ruby
get '/posts', as: :json
```

```
RuntimeError:
 can't modify frozen String
```

This commit fixes the problem by replacing the mutation with a concatenation,
returning a new string.

Then just yield the location for the place where we need some extra processing.

`if !var.nil?` is the same as just `if var`

Currently, if path is a relative path, add format without the discrimination of the query.
Therefore, if there is a query, format at end of the query would been added,
format was not be specified correctly.

This fix add format to end of path rather than query.

Follow up to #21671

- Followup of https://github.com/rails/rails/issues/18693.
- I think we missed deprecating `request_via_redirect` in that pull
  request.
- Originally requested by DHH here
  https://github.com/rails/rails/issues/18333.

- we are ending sentences properly
- fixing of space issues
- fixed continuity issues in some sentences.

Reverts https://github.com/rails/rails/commit/8fc97d198ef31c1d7a4b9b849b96fc08a667fb02 .
This change reverts making sure we add '.' at end of deprecation sentences.
This is to keep sentences within Rails itself consistent and with a '.' at the end.

Similar to 176fbfd6, this makes it possible for other test
frameworks to hook into Rails integration test facilities.

After 9d378747 `ActionDispatch::IntegrationTest`
class is loaded and defined in all Rails environments, not only test but also
production. This is not-intended loading of a class which is only used in
test environment.
To prevent not-intended loading, add `ActiveSupport.run_load_hooks` to
`ActionDispatch::IntegrationTest` with `action_dispatch_integration_test` name
and use it in `ActionMailer`.

This reverts changes made to integration tests in PR #23807.
The issue happens when using capybara with a driver that needs to start a
server in a separate thread like (poltergeist, selenium, etc).
Both threads the capybara server one and the test thread end running
syncronize over the interlock.

These should allow external code to run blocks of user code to do
"work", at a similar unit size to a web request, without needing to get
intimate with ActionDipatch.

Yesterday, when improving how `parsed_body` extracted a parser I wrote
77bbf1e9. Then I thought that was too many changes in one commit
and broke it up locally... or so I thought.

When pushed the extra commits removed the changes! Wups!

In shame, lob those changes together here:

  * 3b94c38a which meant to fix the CHANGELOG syntax error.
  * 5007df5e which meant to mention `parsed_body` in the docs.
  * 036a7a03 which meant to memoize the `parsed_body`.

Little easier to understand when you know the method that's used.

We're not guaranteed to have a `RequestEncoder` to assign on `get` requests
because we aren't extracting the parser from the response content type.

Until now.

When testing:

```ruby
post articles_path, params: { article: { title: 'Ahoy!' } }, as: :json
```

It's common to want to make assertions on the response body. Perhaps the
server responded with JSON, so you write `JSON.parse(response.body)`.
But that gets tedious real quick.

Instead add `parsed_body` which will automatically parse the reponse
body as what the last request was encoded `as`.

Prototype, you have served us well. But you are no longer how we make an
XMLHttpRequest. RIP

Turns

```
post articles_path(format: :json), params: { article: { name: 'Ahoy!' } }.to_json,
  headers: { 'Content-Type' => 'application/json' }
```

into

```
post articles_path, params: { article: { name: 'Ahoy!' } }, as: :json
```

We want to get rid of the `Live::Response` so we are consolidating methods
from `Live::Response` and `Response` by merging them together.

This adds an `#empty` method to the request so we don't need to
hard-code the empty array each time we call an empty
`ActionDispatch::Request`.

The work here is a continuation on combining controller and integration
test code bases into one.

`skip_filter`, `skip_action_callback` may both are deprecated in Rails 5.1 so waring msg should be specific.

Rails 4.x and earlier didn't support `Mime::Type[:FOO]`, so libraries
that support multiple Rails versions would've had to feature-detect
whether to use `Mime::Type[:FOO]` or `Mime::FOO`.

`Mime[:foo]` has been around for ages to look up registered MIME types
by symbol / extension, though, so libraries and plugins can safely
switch to that without breaking backward- or forward-compatibility.

Note: `Mime::ALL` isn't a real MIME type and isn't registered for lookup
by type or extension, so it's not available as `Mime[:all]`. We use it
internally as a wildcard for `respond_to` negotiation. If you use this
internal constant, continue to reference it with `Mime::ALL`.

Ref. efc6dd55

We should be asking the mime type method for the mime objects rather
than via const lookup

Actions are processed through `dispatch`, so they should have the
request set on them before any user land code can be executed.  Lets
stop setting _env on the controller, and give access to it through the
`env` method.

 - webservers do not do it
 - it makes redirect urls ugly when request.host is used for redirection

It's better to use Ruby methods when possible over methods defined by
Active Support because then it does not need to rely on any
dependencies.

Based on #19976 and #18305.

`*args` is not required here and should be avoided when not necessary
because `*args` are slower than `args` and create unnecessary array
allocations.

This method is being used in `#xml_http_request`, but was not properly
required. This causes `NoMethodError` on projects that are doing
integration test.