It makes sense to be as strict as possible
with headers from the outside world,
but allowing @ to support Apache's mod_unique_id
(see #31644) seems OK to me

These assertions did matter due to the inconsistent behavior of
[the #parameters method][1]. Today, it behaves consistently and they
could be removed. Also, one of the methods was stubbed somewhat
incorrectly, so it is better not to stub and instead, make them close
to more realistic use cases.

[1]: https://github.com/rails/rails/pull/13999#issuecomment-34601746

`make_tmpname` was removed by https://github.com/ruby/ruby/commit/25d56ea7b7b52dc81af30c92a9a0e2d2dab6ff27.
In this case, we want a file name, not a `File`. So cannot use `Tempfile`.

Fixes #31458

`BigDecimal.new` has been deprecated in BigDecimal 1.3.3
 which will be a default for Ruby 2.5.

Refer
https://github.com/ruby/bigdecimal/commit/533737338db915b00dc7168c3602e4b462b23503

* This commit has been made as follows:

```
cd rails
git grep -l BigDecimal.new | grep -v guides/source/5_0_release_notes.md | grep -v activesupport/test/xml_mini_test.rb | xargs sed -i -e "s/BigDecimal.new/BigDecimal/g"
```
- `activesupport/test/xml_mini_test.rb`
Editmanually to remove `.new` and `::`

- guides/source/5_0_release_notes.md
This is a Rails 5.0 release notes.

Follow up of #31432.

and defaults to `Digest::MD5`.

Replaced calls to `::Digest::MD5.hexdigest` with calls to `ActiveSupport::Digest.hexdigest`.

Follow up of #31390.

Change the system tests to set Puma as default server only when the user haven't specified manually another server.

Matches Hash#each behaviour as used in Rails 4.

Use Object#deep_dup to safely duplicate policy values

Not everyone uses iTerm2 and whereas Terminal.app on a mac just ignores
that and outputs the path, other terminals like those on Ubuntu do not.
A friendlier default is one that works by default.

Closes #31159
Closes #30957

Fixes #31220.

This now modernizes these tests to use encrypted cookies instead of
using secret_token HMACs. This commit also adds a tests to ensure
session cookies with :expires_after set are invalidated and no longer
accepted when the time has elapsed.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy

https://travis-ci.org/rails/rails/jobs/304428814#L1977

As `@cache_path` is expected to be a directory name, use `Dir.mktmpdir`.
And omit unnecessary `Dir.tmpdir`.

Instead of `Dir::Tmpname.make_tmpname`, an internal method which does not guarantee uniqueness, use `Tempfile.create`.

To be removed in Rails 6.0 (default for the deprecate helper).  Code
moved around as well for the ActiveSupport::Deprecation modules, since
it was dependent on ActiveSupport::Inflector being loaded for it to
work.  By "lazy loading" the Inflector code from within the Deprecation
code, we can require ActiveSupport::Deprecation from
ActiveSupport::Inflector and not get a circular dependency issue.

The Problem
-----------

The following line from `String#camelize`:

  string = string.sub(/^(?:#{inflections.acronym_regex}(?=\b|[A-Z_])|\w)/) { |match| match.downcase }

and the following line from `String#camelize`:

  word.gsub!(/(?:(?<=([A-Za-z\d]))|\b)(#{inflections.acronym_regex})(?=\b|[^a-z])/) { "#{$1 && '_'.freeze }#{$2.downcase}" }#{$2.downcase}" }

Both generate the same regexep in the first part of the `.sub`/`.gsub`
method calls every time the function is called, creating an extra object
allocation each time.  The value of `acronym_regex` only changes if the
user decides add an acronym to the current set of inflections and apends
another string on the the regexp generated here, but beyond that it
remains relatively static.

This has been around since acronym support was introduced back in 2011
in PR#1648.

Proposed Solution
-----------------
To avoid re-generating these strings every time these methods are
called, cache the values of these regular expressions in the
`ActiveSupport::Inflector::Inflections` instance, making it so these
regular expressions are only generated once, or when the acronym's are
added to.

Other notable changes is the attr_readers are nodoc'd, as they shouldn't
really be public APIs for users.  Also, the new method,
define_acronym_regex_patterns, is the only method in charge of
manipulating @acronym_regex, and initialize_dup also makes use of that
new change.

** Note about fix for non-deterministic actionpack test **

With the introduction of `@acronym_underscore_regex` and
`@acronym_camelize_regex`, tests that manipulated these for a short
time, then reset them could caused test failures to happen.  This
happened because the previous way we reset the `@acronyms` and
`@acronym_regex` was the set them using #instance_variable_set, which
wouldn't run the #define_acronym_regex_patterns method.

This has now been introduced into the actionpack tests to avoid this
failure.