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]
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]
pg guide, advanced features [ci skip]
pg guides, some stylistic changes [ci skip]
Merge branch 'postgres/guide'