The subtype will (quite reasonably) ignore the possibility that it has
`changed_in_place?` by becoming nil.

Fixes #19467

No need to merge hashes when simply setting options does the job.

The expected value comes first. Related to #19465.

Fix ActiveModel::Errors#delete return value to stay backward compatible

Rails 5.0 changes to ActiveModel::Errors include addition of `details`
that also accidentally changed the return value of `delete`. Since
there was no test for that behavior it went unnoticed. This commit
adds a test and fixes the regression.

Small improvements to comments have also been made. Since `get` is
getting deprecated it is better to use `[]` in other methods' code
examples. Also, in the module usage example, `def Person.method`
was replaced with a more commonly used `def self.method` code style.

[ci skip] Consistent spacing inside hash brackets in Action View guide.

Deprecate alias_method_chain in favour of Module#prepend

…as discussed #19413

Change 'a' to 'an' for 'HABTM' word [ci skip]

[ci skip] Add link for "parameter_names section"

[ci skip] Change `the null` to `a not null constraint`

 Fix typo in belongs_to_required_by_default config's CHANGELOG entry [ci...

Fix documentation for find_or_create_by

Check subtype limit before using the default limit

As described here https://github.com/rails/rails/issues/19420. When
using the Postgres BigInt[] field type the big int value was not being
translated into schema.rb. This caused the field to become just a
regular integer field when building off of schema.rb. This fix will
address this by delegating the limit from the subtype to the Array type.

https://github.com/rails/rails/issues/19420

Fix ActiveModel::Errors deprecation messages failing when used on its own

The previous change 92a3c8dc improved
this example, but calling .create + #valid? ends up running validations
twice, and we don't want to showcase that.

Lets use the normal use case of building a new object and calling #save
on it, which is what a basic scaffold with Active Record does.

[ci skip]

use `Dir.exist?` instead of deprecated `Dir.exists?`

 Removed requiring ostruct because its unused

[ci skip] Fix typos in test runner's help output

  Deprecation messages in ActiveModel::Errors are using String#squish
from ActiveSupport but were not explicitly requiring it, causing failures
when used outside rails.

The code in the comment fails on concurrent inserts if done inside a transaction. 

The fix is to force a savepoint to run so that if the database raises an unique violation exception. Otherwise, you'll get errors like:

```
   (0.3ms)  BEGIN
  Cart Load (0.5ms)  SELECT  "carts".* FROM "carts"  WHERE "carts"."uuid" = '12345' LIMIT 1

# Another process inserts a cart with uuid of '12345' right now

  SQL (4371.7ms)  INSERT INTO "carts" ("created_at", "updated_at", "uuid") VALUES ('2015-03-21 01:05:07.833231', '2015-03-21 01:05:07.833231', '12345') RETURNING "id"  [["created_at", Sat, 21 Mar 2015 01:05:07 PDT -07:00], ["updated_at", Sat, 21 Mar 2015 01:05:07 PDT -07:00], ["uuid", "12345"]]
PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "carts_uuid_idx1"
DETAIL:  Key (uuid)=(12345) already exists.
: INSERT INTO "carts" ("created_at", "updated_at", "uuid") VALUES ('2015-03-21 01:05:07.833231', '2015-03-21 01:05:07.833231', '12345') RETURNING "id"

# Retrying the find

  Cart Load (0.8ms)  SELECT  "carts".* FROM "carts"  WHERE "carts"."uuid" = '12345' LIMIT 1
PG::InFailedSqlTransaction: ERROR:  current transaction is aborted, commands ignored until end of transaction block
: SELECT  "carts".* FROM "carts"  WHERE "carts"."uuid" = '12345' LIMIT 1
   (0.1ms)  ROLLBACK
ActiveRecord::StatementInvalid: PG::InFailedSqlTransaction: ERROR:  current transaction is aborted, commands ignored until end of transaction block
: SELECT  "carts".* FROM "carts"  WHERE "carts"."uuid" = '12345' LIMIT 1
```

[ci skip] Check a result of `valid?` instead of `create`

This reverts commit 044f9ab7.

Also edited the copy to be more consistent with `rake routes`.

See #19323.

Fix handling of empty X_FORWARDED_HOST header.

Replace occurences of alias_method_chain with their Module#prepend counterpart

Thanks @fbernier for suggestion! <3
At this moment we can use Module#prepend in all all cases
except of Range because of the bug [1] in MRI 2.2
[1] https://bugs.ruby-lang.org/issues/10847