rails/rails#18305

Conflicts:
	guides/source/4_0_release_notes.md

[skip ci] Minor edits for clarity in the test fixture guide.

* Add conjunction for clarity.
* Add comma between independent clauses: create command, and generation
  of fixtures.

Documenting remove_column as a reversible migration method

Avoid loading user's psqlrc when loading test structure

[skip ci] Active Record Validations guide fixes

- Remove deprecated `[]=`

- Fix duplicate `errors#add` example. The second code example was
originally `[]=`, replace it with `[] <<`.

- Improve explanations for `errors#add` and `errors#full_messages`

Follow-up to PR #19457 (closed after borking my git history). Apologies
for the duplicate PR.

cc @kaspth

update `config.assets.digest` default value in guides [ci skip]

[ci skip] Update 4.2 Release notes with more assert_select information.

Adds documentation for substitutions and why Regexp substitions look different
when assertion fails.

Update ActiveSupport::Subscriber docs

This reverts commit 5cfa6a8a, reversing
changes made to bfd5bf83.

Reason: This broken travis build.

DRY up STI subclass logic

the newer method used for discriminating new records did not
use the older and more robust method used for instantiating
existing records, but did have a better post-check to ensure
the sublass was in the hierarchy. so move the descendants check
to find_sti_class, and then simply call find_sti_class from
subclass_from_attributes

For consistency purposes with the changes done in https://github.com/rails/rails/pull/12285

Follow up of https://github.com/rails/rails/pull/19263.

Merge pull request #19452 from pinglamb/fix-referencing-wrong-alias-when-joining-tables-of-has-many-through-association

Fix referencing wrong aliases while joining tables of has many through association

PostgreSQL, Add test case for "Infinity" string assignment to float columns

[ci skip] Remove unacceptable method name

Fix incorrect description for `assert_nothing_raised`. [ci skip]

Small doc fix. [CI SKIP]

Make sure Array#to_sentence always returns a String