Verify that route constraints respond to the expected messages instead of silently failing to enforce the constraint
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 exception when logging SQL w/ nil binary value.
[Fixes #14948] Hash#to_query: Changed a way how empty hash and empty array are serialized

Empty Hash or Array should not present in serialization result

    {a: []}.to_query # => ""
    {a: {}}.to_query # => ""

For more info see #14948.
Fix Formatting in AR Readme

Don't make free form text appear in code blocks [ci skip]
Convert column name to string only once
Remove deprecated file(`active_support/core_ext/object/to_json.rb`) and its test case.
Follow-up to #14905 and formatting pass

on the Getting Started guide.

[skip ci]

Added 2 changes suggested by @robin850 [skip ci]
Merge pull request #14961 from jonatack/patch-7

Follow-up to #14905 and formatting pass [ci skip]
Merge pull request #14974 from kuldeepaggarwal/remove-deprecated-files

Remove deprecated file
Merge pull request #14973 from kuldeepaggarwal/f-refactor

Convert column name to string only once
Merge pull request #14946 from jcoleman/fix-null-binary-column-logging-exception

Fix exception when logging SQL w/ nil binary value.

Conflicts:
	activerecord/CHANGELOG.md
Invert the if/else clause

The case where the value is present is more common
Merge pull request #14949 from bogdan/empty-hash-array-parameterization

[Fixes #14948] Hash#to_query: right serialization for  empty hash and array
No need the else clause
Improve CHANGELOG [ci skip]
Merge pull request #11166 from xavier/callable_constraint_verification

Callable route constraint verification

Conflicts:
	actionpack/CHANGELOG.md
Do not use short-circuit return