Verify that route constraints respond to the expected messages instead of silently failing to enforce the constraint
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.
Remove tests method for test cases when controller can be inferred.
Merge pull request #14966 from tgxworld/remove_unnecessary_setting_of_controller_name

Remove tests method for test cases when controller can be inferred.
refactor, move `column_for` to `AbstractAdapter` for better reuse.
Document ActionController::TestCase::Behavior#process

[ci skip]
Merge pull request #14923 from gaurish/document-testcase-process

Document ActionController::TestCase::Behavior#process

[ci skip]
Rails One Click is not maintained anymore [ci skip]

Let's tell people that they should rely on Tokaido instead of Rails One
Click.

Fixes #14960.
Convert column name to string only once
Simplify unless conditional with OR
Get rid of conditional since column_for handles raising now

column_for will raise in case column is not found for the given table,
so there is no need to handle that here.
Simplify building options hash in rename column method for mysql
Use the reader attribute rather than the instance var when possible
Fix examples indent and improve #process docs a bit [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]