This resulted in some unrelated errors on Traivs:

  1) Error:
PostgresqlUUIDTest#test_treat_blank_uuid_as_nil:
ActiveRecord::StatementInvalid: PG::UndefinedColumn: ERROR:  column "thingy" of relation "uuid_data_type" does not exist
LINE 1: INSERT INTO "uuid_data_type" ("thingy") VALUES ($1) RETURNIN...
                                      ^
: INSERT INTO "uuid_data_type" ("thingy") VALUES ($1) RETURNING "id"

/cc @zuhao

Mismatch branch name between text and code example [ci skip]

[ci skip]

Dup the changed_attributes otherwise we could lose them

Use Rails::Paths::Path#existent in database_configuration

Database configuration was trying to load the first path that
config.paths['config/database'] was returning even if the path didn't
exist in the filesystem.

Because Rails::Paths::Path has the possibility to return more than 1
path (as an array), database_configuration should filter down the paths
to the existing one and then load the first one.

This would make it possible to move the database.yml file and add the
new path to paths['config/database'] and still load the configurations.

Closes #14989

Fix Hash#deep_merge bug and improve documentation — resolves #12738

[related #13166]

[Regression 3.2 -> 4.0] Fix bugs with changed attributes tracking when transaction gets rollback

HTTP::Headers#key? correctly converts

Previously if you were looking for a given key, the header may incorrectly tell you that it did not exist even though it would return a valid value:

```ruby
env     = { "CONTENT_TYPE" => "text/plain" }
headers = ActionDispatch::Http::Headers.new(env)
headers["Content-Type"] 
# => "text/plain"

headers.key?("Content-Type")
# => false
```

This PR fixes that behavior by converting the key before checking for presence

This case prevents against regressions. The change was suggested in a recent
PR but the all our tests passed.

typo fixes in active_record_basics.md [skip ci]

WIP: Make ActionMailer tests order-independent.

rename var to fix overshadowing of variable

Follow-up to #14990 [ci skip]

Rewrite to avoid 'we'/'you', add missing period, and keep lines at 80 chars. Cheers :)

Improve readability with help from @senny

[ci skip]

[ci skip] Add feature section and normalize prologue with other guides.

Also change Json => JSON.

Fill Ruby's Range & IPAddr link.

o_O

fix guides mobile nav issue

Include label value in i18n attribute lookup

Ensure that .process return the method return

Previously, only the object and method name from the label tag were
used when looking up the translation for a label. If a value is
given for the label, this ought to be additionally used. The
following:

    # form.html.erb
    <%= form_for @post do |f| %>
      <%= f.label :type, value: "long" %>
    <% end %>

    # en.yml
    en:
      activerecord:
        attributes:
          post/long: "Long-form Post"

Used to simply return "long", but now it will return "Long-form
Post".

* Strips leading underscores.
* Changes some unnecessary gsub!s to sub!s.
* Replaces some anchors ^, $ with \A, \z.
* Documents that human inflection rules are applied.
* Documents that words are downcased except acronyms.
* Adds an example with an acronym.
* Rewords docs.