Add test case and documentation for skip_before_filter.

Add prepend option to protect_from_forgery.

Remove ActionController::HideActions (closes #18336)

https://github.com/rails/rails/commit/5937bd02dee112646469848d7fe8a8bfcef5b4c1#commitcomment-9205786

The new test/docs further explain the conflicts that can happen when
mixing `:if`/`:unless` options with `:only`/`:except` options in
`skip_before_action`.

The gist is that "positive" filters always have priority over negative
ones.

The previous commit already showed that `:only` has priority over `:if`.

This commit shows that `:if` has priority over `:except`.

For instance, the following snippets are equivalent:

```ruby
skip_before_action :some_callback, if: -> { condition }, except: action
```

```ruby
skip_before_action :some_callback, if: -> { condition }
```

Test case for using skip_before_filter with the options :only and :if
both present. In this case, the :if option will be ignored and :only
will be executed.

Closes #14549 (the commit was cherry-picked from there).

Enqueued email assertions

Add :only option to assert_enqueued_jobs

With the option, assert_enqueued_jobs will check the number of times a specific kind of job is enqueued.

Fix potenital infinite recursion in changed_for_autosave?

Fix count on a separate connection

Previosly count and other AR calculations would convert
column_name_for_operation to sql on a default Arel::Table.engine
(AR::Base) connection. That could lead to trouble if current
model has a connection to a different adapter or Base connection
is inaccessible.

Overwrite descriptions generated by Generators::Base to respect acronym.

Add _mailer suffix to generated file names.

The build has failed when running the date/time ext tests in isolation
due to the missing extension, so better than adding a require is using
just Ruby in this case.

https://travis-ci.org/rails/rails/jobs/46107954#L1077

fix `attribute` method scoping(i.e. private)

Changing sets -> set [ci-skip]

Updating some minor grammar issue.

Add #weekend?, #next_weekday, and #prev_weekday methods to Date, Time, and DateTime

ref #52f64126

Following the same naming convention used in
controllers and jobs.

Add log_level options to configuration guide

replacing 'attribute' method with an alias

 Fixed typos and wording in ActiveRecord CHANGELOG [ci skip]

Fix lookup of fixtures with non-string(like Fixnum) label

 - Fixtures with non-string labels such as integers should be accessed
   using integer label as key. For eg. pirates(1) or pirates(42).
 - But this results in NotFound error because the label is converted into string before
   looking up into the fixtures hash.
 - After this commit, the label is converted into string only if its a
   symbol.
 - This issue was fount out while adding a test case for
   https://github.com/rails/rails/commit/7b910917.

Add #prev_day and #next_day as counterparts to #yesterday and #tomorrow for Date, Time, and DateTime