[ci skip]

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

[ci skip]

[ci skip]

```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

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

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.

[ci skip]

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.

Follow up to 333bfd89

Mainly around `nil`

[ci skip]

This method has the same behavior as `Hash#merge!`, returns current
`ActionController::Parameters`.

* ActionController::Parameters#deep_dup

* Tests for ActionController::Parameters#deep_dup

* Fix test for ActionController::Parameters#deep_dup

* More tests for ActionController::Parameters#deep_dup

[Rafael Mendonça França + Pavel Evstigneev]