Since https://github.com/rails/arel/pull/337 Oracle adapter uses better
top N query using `fetch first n rows only`, which can remove this
unless condition.

* This commit passes with Oracle database
```ruby
$ ARCONN=oracle bin/test test/cases/relation/delete_all_test.rb -n test_delete_all_with_order_and_limit_deletes_subset_only
Using oracle
Run options: -n test_delete_all_with_order_and_limit_deletes_subset_only --seed 1081

.

Finished in 8.068626s, 0.1239 runs/s, 0.6197 assertions/s.
1 runs, 5 assertions, 0 failures, 0 errors, 0 skips
$
```

* SQL statement includes `ORDER BY` and `FETCH FIRST n ROWS ONLY`

```sql
  Post Destroy (12.5ms)  DELETE FROM "POSTS" WHERE "POSTS"."ID" IN (SELECT "POSTS"."ID" FROM "POSTS" WHERE "POSTS"."AUTHOR_ID" = :a1 ORDER BY "POSTS"."ID" ASC FETCH FIRST :a2 ROWS ONLY)  [["author_id", 1], ["LIMIT", 1]]
```

* activesupport(class_attribute): Use redefine_singleton_method

* activesupport(class_attribute): Use keyword arguments

* activesupport(class_attribute): Avoid unnecessary redefinition for default

Add test and change how format set in ActionMailer

Previously this used self.formats= to set the format which render would
use to find templates. This worked, but was untested, and looked a
little confusing because it was doing the mutation within a loop.

This commit replaces the assignment with passing formats: [format] into
the render call, which makes it more obvious that that's the purpose of
the format. It also adds a test to verify the formats being used.

Allow nil format on templates

This was using a model without a default scope what made the test don't
break anymore if the code is removed.

Relax table name detection in `from` to allow any extension like INDEX hint

Enable SQL statement cache for `find` on base class as with `find_by`

#35360 allows table name qualified if `from` has original table name.
But that is still too strict. We have a valid use case that `from` with
INDEX hint (e.g. `from("comments USE INDEX (PRIMARY)")`).
So I've relaxed the table name detection in `from` to allow any
extension like INDEX hint.

Fixes #35359.

Add reselect method

`require "bcrypt"` is unsed since #15431.
`require "models/topic"` is unused since 893c647d.

Related and follows d333d852.

This is an alternative of 65c4b1b5.

https://buildkite.com/rails/rails/builds/59147#117e9445-23e8-455a-b486-ea0ae9636405/120-129

Some untransactional tests (e.g. `CallbacksOnMultipleActionsTest`) makes
`topics` table dirty.
We should reset dirty `topics` table before `SerializedAttributeTest` is
run.

https://travis-ci.org/rails/rails/jobs/499719624#L1209-L1215

A string S matches ([.]|\b)html if an only if matches \bhtml:

  * If S matches [.]html, then it matches \bhtml.

  * If S matches \bhtml, then it matches \bhtml.

Reciprocally:

  * If S matches \bhtml, then it matches ([.]|\b)html.

The character class can be removed, and since we are on it we remove the
group too so that it is clear to a reader of the code that there is no
grouping going on.

References #35166.

[ci skip] The `find` method coerces the given arguments to integer if the `primary key` is integer

[ci skip] Fix the documentation of ActiveRecord::FinderMethods#find

It's mentioned everywhere as `ActiveRecord::RecordNotFound` so to be
coherent with the rest of the documentation I've applied it here.
Also doc was saying if the parameter is integer it coerces it which is
other way around.

This fixes both #34555 and #34738.

Revert "Merge pull request #34900 from gmcgibbon/fix_test_find_only_some_columns"

This reverts commit ff807f82, reversing
changes made to 9f1a07af.

Revert "Merge pull request #34560 from gmcgibbon/fix_decorate_leak_on_serial_attr_test"

This reverts commit bd623893, reversing
changes made to ec66c6a2.

Revert "Fix unstable `test_serialized_attribute_works_under_concurrent_initial_access` test"

This reverts commit 65c4b1b5.

Add note about idle connection reaper to 5.2 release notes

https://travis-ci.org/rails/rails/jobs/499061043#L1187-L1193

Run activesupport's memcache store tests on Buildkite

Related to 287920ca

When the middle association doesn't have any records and the inner
association is not an empty scope the owner will be `nil` so we can't
try to reset the inverse association.

[ci skip]

Add negative scopes for all enum values

Fix code examples' indentation in ActionDispatch::Routing

Pass locals in to the template object on construction

Caching `find_by` statements on STI subclasses is unsafe, since
`type IN (?,?,?,?)` part is dynamic, and we don't have SQL statements
cache invalidation when a STI subclass is created or removed for now.

Also, max precision (= 6) isn't regarded as unlimited precision for now.