Simplify `find_by_statement_cache` interaction down to a class-level ivar with a single accessor `cached_find_by_statement`.

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.

 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

Don't autosave unchanged has_one through records

Conflicts:
	activerecord/CHANGELOG.md

Allow YAML serialization when using TZ aware attributes

Added method `#eql?` to `ActiveSupport::Duration`, in addition to `#==`.

Conflicts:
	activesupport/CHANGELOG.md
	activesupport/lib/active_support/duration.rb
	activesupport/test/core_ext/duration_test.rb

ActiveRecord deprecated finders are broken in 4.2

Ruby 2.2 is near to be released so we should support it properly