* 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]

`ActionController::Parameters#merge` call `HashWithIndifferentAccess#merge`.
In addition, it calls `HashWithIndifferentAccess#update` from
`HashWithIndifferentAccess#merge`,  where it is called the `#to_hash` of argument.
But `ActionController::Parameters#to_hash` is deprecated, warning message is
displayed.
To avoid this, modify to convert object to `Hash`.

Fixes #26415

Makes it clearer why we're doing this
Add a comment to elaborate on what the undef means and move it closer
to the deprecations so it's less likely for whoever pulls the
deprecations that it should go too.

Style/SpaceBeforeBlockBraces
Style/SpaceInsideBlockBraces
Style/SpaceInsideHashLiteralBraces

Fix all violations in the repository.

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.

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

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.

Trust `Object#dup` in `ActionController::Parameters`, using `#initialize_copy` to manually duplicate the underlying parameters hash

It looks like `ActionController::Parameters#dup` is leftover from when the class inherited from `Hash`. We can just trust `#dup`, which already copies the `@permitted` instance variable (confirmed by tests). We still define a `#initialize_copy` to make `@parameters` a copy that can be mutated without affecting the original instance.

When `ActionController::Parameters` is duplicated with `#dup`, it doesn't create a duplicate of the instance variables (e.g. `@parameters`) but rather maintains the reference (see <http://ruby-doc.org/core-2.3.1/Object.html>). Given that the parameters object is often manipulated as if it were a hash (e.g. with `#delete` and similar methods), this leads to unexpected behaviour, like the following:

```
params = ActionController::Parameters.new(foo: "bar")
duplicated_params = params.dup
duplicated_params.delete(:foo)

params == duplicated_params
```

This fixes the bug by defining a private `#initialize_copy` method, used internally by `#dup`, which makes a copy of `@parameters`.

In the docs: "+permit_all_parameters+ - If it's +true+, all the parameters will
be permitted by default. The default is +false+."

The changes in this commit are twofold:

 1. The examples showing `#require` accepting two arguments were wrong - you
    have to wrap the arguments (two, or more) in an array.
 2. `ActionController::Parameters` has an `#inspect` method now (since
    https://github.com/rails/rails/pull/23732), and the documentation should
    reflect that.

This example code wasn't getting wrapped in a `<code>` tag due to incorrect indentation.

This brings the behavior more inline with other similar cases, such as
receiving a hash when an array of scalars was expected. Prior to this
commit, the key would be present, but the value would be `nil`

- Added missing `"`.

Add explanation about accepts_nested_attributes_for keys in the strong parameters documentation [skip ci]

[ci-skip]

This method will only be added when used with Ruby 2.3.0 or greater.
This method has the same behavior as `Hash#dig`, except it will convert
hashes to `ActionController::Parameters`, similar to `#[]` and `#fetch`.

- Fixes #23822.

Creating a protected getter method for `@parameters`.

While iterating an AC::Parameters object, the object will mutate itself
and stick AC::Parameters objects where there used to be hashes:

  https://github.com/rails/rails/blob/f57092ad728fa1de06c4f5fd9d09dcc2c4738fd9/actionpack/lib/action_controller/metal/strong_parameters.rb#L632

If you use `permit` after this iteration, the `fields_for_style` method
wouldn't return true because the child objects are now AC::Parameters
objects rather than Hashes.

fixes #23701

Now that AC::Parameters is no longer a Hash, it shouldn't look like a hash.

`NEVER_UNPERMITTED_PARAMS` is deprecated in Rails 4.2. See #15933.

and remove unecessary spaces in string interpolation.

See bug #21032.

It's reasonable to expose different value readers.

We can provide a more flexible upgrade experience by warning users they are using unsafe methods instead of forcing the safe API by deprecating before removal. This PR provides this functionality.

Fixes #23026

See discussion at #23026