add include_hidden option to collection_check_boxes helper
Save has_one associations only if record has changes

Prevents save related callbacks such as `after_commit` being
triggered when `has_one` objects are already persisted and have no
changes.
Only use BINARY for mysql case sensitive uniqueness check when column has a case insensitive collation.
typos fixes
Tweak Configuring Guide language

the fact that Rails is multi-thread by default is not very relevant. see https://github.com/rails/rails/pull/13998 [ci skip]
:scissors:

This commit also addresses rails/docrails#169 and rails/rails#14159
Improve guide for Hash#transform_keys and related methods.

[skip ci]
Fix code alignment in duplicable? guide.

[skip ci]
Improve collision explanation for Hash#transform_keys.

Based on comments at
https://github.com/rails/docrails/commit/42417b0013819e573be9ffc5402e179003156e8e#commitcomment-5527251

[skip ci]
Refactor tests from BaseRackTest into BaseRequestTest
Fix warnings due to:
- unused variable in PG Adapter.
- Ambiguous argument warning from range_test for use - to + Infinity range without brackets.
Merge pull request #14248 from vipulnsward/fix-pg-ar-warnings

Fix PG warnings.
Merge pull request #14239 from zzak/refactor_actionpack_dispatch_request_tests

Refactor actionpack ActionDispatch::Request tests
fixes default attributes for button_tag
fix accept nil options
fix button tag without options
fix form button
fix indentations
Add a comment to ensure that a test won't be removed [ci skip]

The 'cow' => 'kine' inflection has gone with c300dca9 but it should
not be removed from the tested irregularities since it ensures that
inflections work with words that do not begin with the same letters.
fix content_or_options to be replaced by options and set to nil