`Mutex` was removed at 8eb7561a.

Use :git for specifying dependencies from github with https protocol

use correct Gemfile in `bin/setup` test

Do not mutate AC::TestRequest::DEFAULT_OPTIONS

Currently, `bin/setup` test uses Gemfile of Rails. But this Gemfile is not a
file to be used in Rails application.
Add a Gemfile to Rails application that is created for test, it has been
modified to use the Gemfile.

Remove outdated "#TODO: Fix for binds." comment [ci skip]

This comment was added at eaf54865 but already implemented.

- This is required for bundler 1.13.0 because of which lot of specs are
  failing on Travis CI.
- Similar to https://travis-ci.org/rails/rails/jobs/158905576#L559

Pass `set_inverse_instance` block to `sc.execute` for `SingularAssociation`

Fix table name typos [ci skip]

update doc about default `cache_store` [ci skip]

The default `cache_store` has changed in 8f0e0b63 to use `file_store`.

call `.to_h` to avoid using deprecated method

Clarify expires_now documentation 

`ActionController::Parameters#merge` call `HashWithIndifferentAccess#merge`.
In addition, it calls `HashWithIndifferentAccess#update` from
`HashWithIndifferentAccess#merge`,  where it is called the `#to_hash` of argument.
But `ActionController::Parameters#to_hash` is deprecated, warning message is
displayed.
To avoid this, modify to convert object to `Hash`.

Fixes #26415

[ci skip]

[ci skip] Simply formatting documents

Revert " [ci skip] Remove duplicate example."

 [ci skip] Remove duplicate example.

Selectively add `li ul, li ol { margin:0 1.5em; }` to subColl as needed

Remove the word "mongrel" from documents [ci skip]

Currently mongrel is not maintained.
And it couldn't be built with any Ruby versions that
supported by Rails.

It is reasonable to remove the word "mongrel" in order to avoid
confusion from newcomer.

Fix Remaining Case-In-Assignment Statement Formatting

Minor grammar fix in engines.md [ci skip]

Recently, the Rails team made an effort to keep the source code consistent, using Ruboco
(bb1ecdcc and below). Some of the case
statements were missed.

This changes the case statements' formatting and is consistent with changes
in 810dff7c and db63406c.

Remove redundant `!loaded?` condition

Remove not used alias method

Bang methods of `AR::QueryMethods` are used only internally.
We only use `left_outer_joins!`, so we can remove this alias.

Already checked `if !find_target? || loaded?`, unnecessary `!loaded?` in
elsif condition.

Move `require "models/post"` before `require "models/comment"`

use `descendants` to get class that inherited `ActiveJob::Base`

Remove unnecessary `query_scope`

change `Class#descendants` to public API [ci skip]

`Class#descendants` has already been displayed in Rails guide,
so I think that may be displayed in doc.
http://guides.rubyonrails.org/active_support_core_extensions.html#descendants