Refactor AR's validations_test.rb

The `:context` switch feature is implemented in
ActiveRecord::Validations#valid? method, so we should rename the test
names, and execute `valid?` in the test.

Change test name in AR's validations_test.rb

This test is testing save method's code
Fix bugs with changed attributes tracking when transaction gets rollback
Fix Hash#deep_merge bug and improve documentation — resolves #12738

Previously merging into a hash with a falsy value would not result in
the merge-block being called. The fix is simply to check for presence
of the key in the hash.

The documentation example for `deep_merge` now appropriately
demonstrates what a deep merge does.
Fix Formatting in AR Readme

Don't make free form text appear in code blocks [ci skip]
Arel master is 6.0.0
Get rid of extra local var that does not add to the logic

There are too many "action name" variables around the process method.
Get rid of unused method in AR Post test model
Put back Relation#join method as a delegate to Array

This is a regression 4.0 -> 4.1 fix.
In 4.1.0 Relation#join is delegated to Arel#SelectManager.
In 4.0 series it is delegated to Array#join

This patch puts back the behaviour of 4.0
Merge pull request #14978 from bogdan/relation-join

[Regression 4.0 -> 4.1] Put back Relation#join method as a delegate to Array
Improve CHANGELOG entry [ci skip]
Add support for module-level table_suffix in models

This makes table_name_suffix work the same as table_name_prefix when
using namespaced models. Pretty much the same as 67d1cec.
Merge pull request #14951 from jenrzzz/module_table_name_suffix

Add support for module-level table_suffix in models
Guides: output valid HTML5

[ci skip]
Refactor AssociationScope#get_bind_values
Added #join_id_for(owner) to reflection to avoid accessing the source_macro
Fix rails dbconsole for jdbcmysql adapter.
refactor and clean up delete_all method

Now that delete_all with destroy or delete_all dependency behave
the same we no longer need this conditional. This means we can remove
the new delete_all_with_dependency method I added and go straight
to delete_records from delete_all, passing :all and the dependent
directly.
Merge pull request #14985 from eileencodes/refactor-delete-all-method

refactor and clean up delete_all method
Merge pull request #14984 from jkingdon/add-jdbcmysql

Fix rails dbconsole for jdbcmysql adapter
Merge pull request #12667 from nateberkopec/make_guides_html_valid

Fix Guide HTML validation
pg guide, datatypes and examples. [ci skip]