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.
Don't load test env by default
Only use BINARY for mysql case sensitive uniqueness check when column has a case insensitive collation.
[Active Record] Renamed private methods create_record and update_record

 This is to ensure that they are not accidentally called by the app code.
 They are renamed to _create_record and _update_record respectively.
 Closes #11645
Pass a base relation to build_default_scope when joining

This allows the default scope to be built using the current table alias.
Resolves #12770
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