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`env` is undefined.

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- This PR adds the `reverse_merge` and `reverse_merge!` method to `ActionController::Parameters`
- Fixes #28353

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```diff
diff --git a/actionpack/lib/action_controller/metal/redirecting.rb b/actionpack/lib/action_controller/metal/redirecting.rb
index a3159c29dd..1836a07d4e 100644
--- a/actionpack/lib/action_controller/metal/redirecting.rb
+++ b/actionpack/lib/action_controller/metal/redirecting.rb
@@ -50,8 +50,8 @@ module Redirecting
     #   redirect_to post_url(@post), status: 301, flash: { updated_post_id: @post.id }
     #   redirect_to({ action: 'atom' }, alert: "Something serious happened")
     #
-    # Statements after redirect_to in our controller get executed, so redirect_to doesn't stop the execution of the function.
-    <U+2028># To terminate the execution of the function immediately after the redirect_to, use return.
+    # Statements after +redirect_to+ in our controller get executed, so +redirect_to+ doesn't stop the execution of the function.
+    # To terminate the execution of the function immediately after the +redirect_to+, use return.
     #   redirect_to post_url(@post) and return
     def redirect_to(options = {}, response_status = {})
       raise ActionControllerError.new("Cannot redirect to nil!") unless options
```

https://travis-ci.org/rails/rails/jobs/207908041#L549

Should be `ActionController::ParameterMissing` and not
`ActionController::MissingParameter`.

Corresponding change was done in guides in
https://github.com/rails/rails/pull/9816.

Closes #28033

These files are not using `strip_heredoc`.

Closes #27976

ActionMailer::Base#instrument_name and
ActionController::Base#instrument_name will be frequently called once
caching is enabled. So it's better to freeze them instead of create new
string on every call.

Also, the instrument name in #instrument_fragment_cache will usually
be "write_fragment.action_controller" or
"read_fragment.action_controller". So freezing them might also gain some
performance improvement. We have done something like this in other places:
https://github.com/rails/rails/blob/master/actionview/lib/action_view/template.rb#L348

Prevents PATH_INFO from being used to infer the request format in later
test requests when no explicit format is given.

As the request PATH_INFO may be set before a request, it can't be
deleted during pre-request scrubbing.

Fixes #27774

Revert "significant speedup of AC::Parameters#permit" [26dd9b26] as it broke Parameters#to_h on at least fields_for-style nested params.

it has some methods that override the accessors and calls the original accessors via `super`

this partially reverts 9360b6be

because Struct.new returns a Class, we just can give it a name and use it directly without inheriting from it

`ActionController::Renderer.defaults` was removed in 2db7304c

This reverts commit 5eff7a9c, reversing
changes made to 5f03172f.

Reason: It caused a regression. The test case is on the PR.

Regression introduced by ae291421 / 8363b879.

Previously, cookies were only updated on `GET` requests. Now we will
update the helper for all requests, as part of `process`. Added
regression tests for all available HTTP method helpers in
`ActionController::TestCase`.

Actually, private methods cannot be called with `self.`, so it's not just redundant, it's a bad habit in Ruby

Provide an API interface similar to how format is handled in
Controllers. In situations where variants are not needed (ex: in
Action Mailer) the method will simply trigger a no-op, and will not
affect end users.

This commit changes `parameter_encoding` to `skip_parameter_encoding`.
`skip_parameter_encoding` will set encoding on all parameters to
ASCII-8BIT for a given action on a particular controller.  This allows
the controller to handle data when the encoding of that data is unknown,
for example file systems or truly binary parameters.

If it is explicitly cleared (e.g., response.sending_file = true), then
we should not try to set it again.

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CONENT_LENGTH setted by string length, which is equal to number of
characters in string but StringIO.length is byte sequence and
when payload contains non-ASCII characters, stream's length will be
different. That's why real byte length should be used for CONTENT_LENGTH
header.

Add unit test for CONTENT_LENGTH header fix

It just passes non-ascii symbols as parameters and verifies that
"CONTENT_LENGTH" header has content bytes count as value.

This alternative case expressions read better for my taste, and look more uniform
in a file where other similar case expressions are used (without dynamic clauses).

The current implementation of AC::Parameters#permit builds permitted hashes and
then calls permit! on them.

This filtering is recursive, so we call permit! on terminal branches, but then
ascendants call permit! on themselves when the recursion goes up the stack,
which recurses all the way down again because permit! is recursive itself.
Repeat this for every parent node and you get some scary O-something going on
that I don't even want to compute.

Instead, since the whole point of the permit recursion is to build permitted
hashes along the way and at that point you know you've just come up with a
valid filtered version, you can already switch the toggle on the spot.

I have seen 2x speedups in casual benchmarks with small structures. As the
previous description shows, the difference in performance is going to be a
function of the nesting.

Note that that the involved methods are private and used only by permit.