Merge pull request #16739 from y-yagi/patch-6

[ci skip] Fix example code of ActiveRecord::FixtureSet.context_class
Revert "Merge pull request #16254 from zuhao/refactor_activerecord_attribute_decorators_test"

This reverts commit 16fe19831548f108c113094d106663497fc190d5, reversing
changes made to 4c81c8ce533896be28bdc0b055ff30bb9dee1316.

The call to `Model.reset_column_information` was to express the intent of the test.
`reset_column_information` itself can trigger SQL queries because it checks
for a tables existance. Let's move it outside of the block.

/cc @sgrif
[ActiveJob] extract JobBuffer from helper
Merge pull request #16740 from seuros/activejob

[ActiveJob] extract JobBuffer from helper
Don't test PostgreSQL implementation details

Asserting that the message contains "QUERY PLAN" is enough for the adapter's test.
The plan may vary according to number of tuples etc. but that's out of our concern.
Some documentation fixes [ci skip]
Make test:isolated run without bundler for Action Mailer

Action Mailer tests weren't able to run in isolation without the bundle
exec prefix since we were requiring gems before requiring abstract_unit.

We don't need the `gem` call thus and the require_relative since the
test directory should be present in the load path when we run any test.
Avoid skipping Sidekiq tests on Rubinius

Now that Travis provides support for Rubinius 2.2.10, we can run the
Sidekiq tests against this version.
Merge pull request #16742 from robin850/rbx-aj-sidekiq

Avoid skipping Sidekiq tests on Rubinius
Merge pull request #16741 from robin850/am-isolated-wo-bundle

Make test:isolated run without bundler for Action Mailer
Don't allow AR::Tasks::DatabaseTasks.migrate to mutate Migration.verbose value

This fixes random test failure in migrator_test.rb
Fixed Active Job Backburner adapter
Merge pull request #16746 from cristianbica/fix-backburner-adapter

Fixed Active Job Backburner adapter
Merge branch 'master' of github.com:rails/rails
Added yield to Object#presence
Update examples to show real worth
Use instance_eval on @tenderlove's suggestion :trollface:
Clarify the origin of this great addition to Rails :trollface: :trollface :trollface:
We tenderized the wrong method! Object#try already had the yield option, just needed some tenderloving instance_eval to fit the bill
reduce object allocations in utc_offset

`try` allocates an array on every call, we should avoid calling it in
hotspots.  This reduced AttributeMethodsTest#test_setting_time_zone_aware_attribute_with_string
from 18k allocations to 14k