Right now BenchmarkCleaner allocates hundreds of strings on every request with an exception. This patch moves those strings to be generated at boot once and re-used.

## Bench Methods

I took a modified form of CodeTriage https://github.com/schneems/codetriage-ko1-test-app/blob/master/perf.rake and ran given rake tasks with and without the patch to BacktraceCleaner.

I made an endpoint results in exception

```
  def index
    raise “foo"
  end
```

The gem `memory_profiler` was used to capture objects allocated for a single request. Then `benchmark/ips` was used to test the speed of the patch.

You will see that we use less objects and the code becomes measurably faster with this patch.

## With patch:

Memory for one request:

```
Total allocated 7441
Total retained 37
```

Requests per second:


```
Calculating -------------------------------------
                 ips         4 i/100ms
-------------------------------------------------
                 ips       43.0 (±4.7%) i/s -        216 in   5.037733s
```


## Without patch:

Memory used for one request:


```
Total allocated 11599
Total retained 35 
```

Requests per second:

```
Calculating -------------------------------------
                 ips         3 i/100ms
-------------------------------------------------
                 ips       39.4 (±7.6%) i/s -        198 in   5.052783s
```

## Analysis

The patch is faster:

```
(43.0 - 39.4 ) / 39.4 * 100
# => 9 # % ~ speed bump
```

It also allocates less objects:

```
11599 - 7441
# => 4158
```

These strings are allocated on __EVERY SINGLE REQUEST__. This patch saves us 4158 objects __PER REQUEST__ with exception.

Faster errors == Faster applications

Fix false assertions in AR::TestCase::AttributeMethodsTest

This test has always been green because it uses "assert" and the first
argument is an truthy class/object.

Document that the default for `rails server -b` has changed

Fixes #16578

Standardize on `Rails.application` [ci skip]

[ci skip] Fix typo in activerecord/test/cases/attribute_methods_test.rb:737

[ci skip] modify the description of production log level default

ActiveRecord CHANGELOG improvements [skip ci]

The hash is now string-keyed, and [_]reflect_on_association calls `to_s` on the
argument anyway.

For now, we will just skip the cache when a non-column key is used in the hash.
If the future, we can probably move some of the logic in PredicateBuilder.expand
up the chain to make caching possible for association queries.

Closes #16903
Fixes #16884

This is pretty lame though, so feel free to send a PR with a better fix! ;)

cc @zev @tenderlove

Update RecordNotFound exception cases to include a message with the

[ci skip] Reword a comment in concern's documentation

Nothing biggie. Skimmed through `ActiveSupport::Concern` docs these days
and this one comment seemed a bit off.

This seems to be the style settled on in most of the templates.

 Updated comment about features of fixtures

[ci skip] ActionRecord CHANGELOG docs fixes

Make delegation work with the reserved words passed to `:to`

 - Added a note about some features which are not dependent on
   auto-generated ID's.
 - [ci skip]

Fixes #16956.

Specify dummy app's db migrate path in plugin's test_helper.rb

Conflicts:
	railties/CHANGELOG.md

If code is not eager loaded constants are loaded on demand. Constant
autoloading is not thread-safe, so if eager loading is not enabled
multi-threading should not be allowed.

This showed up in certain Capybara scenarios: Most Capybara drivers
other than Rack::Test need a web server. In particular, drivers for
JavaScript support. Capybara launches WEBrick in its own thread for
those but that per se is fine, because the spec thread and the server
thread are coordinated.

Problem comes if the page being served in the spec makes Ajax calls.
Those may hit WEBrick in parallel, and since WEBrick is multi-threaded
and allow_concurrency? returns true in the test environment before
this patch, threads are spawned to serve those parallel requests. On
the other hand, since eager_load is false by default in the test
environment, constants are not preloaded.

So the suite is autoloading constants in a multi-threaded set. That's
a receipt for paracetamol. The symptom is random obscure errors whose
messages point somehow to constant autoloading.

As a consequence of this fix for allow_concurrency? WEBrick in
Capybara scenarios no longer runs in multi-threaded mode.

Fixes #15089.

Use #inject over #sum to build PG create DB statement

While investigating #16951 I found that another library's monkey-patching of
`Enumerable` was causing the test migrations helper to break when trying to
build the `CREATE DATABASE` statement. The prior approach used `#sum` to build
the string from the options hash.

As the code that combines the options to build the database statement is not
user-facing, using `#inject` here instead will remove the only place where the
database creation/migration code is dependent on ActiveSupport's monkey-patching
of `Enumerable`.

[ci skip] ActionView and ActionMailerCHANGELOG docs fixes

[ci skip] ActiveModel CHANGELOG docs fixes

Grammar correction in ActiveSupport CHANGELOG [ci skip]

[ci skip] ActiveSupport CHANGELOG fixes

1. spacing issues
2. spelling correction
3. grammar correction
4. Add missing docs

Added Responders docs, made Web Console docs more consistent. [ci skip]

[ci skip] AJ docs fixes

1. Indentation
2. Spaces issues
3. Spelling correction
4. Grammar correction
5. Add #:nodoc: to all internal classes