`AttributeSet#dup` has all the behavior we need.

Add `Hash#map_values` to ActiveSupport to simplify a common pattern

Didn't get a chance to convert existing code, I'll skim through the code
base to make use of this later this afternoon.

[ci skip]

Rename options param of #time_ago_in_words to match API change

Always pass a column with a cast_type to quote

Add punctuation and fix capitalization in Getting Started guide

[ci skip] Fix router option reference in 4.2 release notes.

Fix more small grammar mistakes in the [Getting Started guide](http://edgeguides.rubyonrails.org/getting_started.html#security), in sections 9 and 10.

Specifically, within sections 9.1 “Basic Authentication”, 9.2 “Other Security Considerations”, and 10 “What's Next?”.

“Rails Guides” is generally capitalized on the site, including on the linked page and in the title of the site, so I capitalized it.

“The” is not part of the name of the Ruby on Rails Security Guide – “The” is not in the title on that page.

And I added a missing period and a missing colon at the ends of two sentences.

The only case where we got a column that was not `nil`, but did not
respond to `cast_type` was when type casting the default value during
schema creation. We can look up the cast type, and add that object to
the column definition. Will allow us to consistently rely on the type
objects for type casting in all directions.

Rename `include_seconds_or_options` to `options` to match 6b9356a4 (which removed the deprecation introduced by #6077).  This has no functional impact because the parameter is passed directly through, but makes it clearer that the parameter no longer supports a boolean as input.

[Postgresql tests] remove duplicate helper

Update the levenshtein distance method in guides.

Add missing period in Getting Started guide

Within section 5.10 Adding Some Validation

MySQL doesn't work with foreign keys

This was previously fixed in e84799d9 but broken in 3f596f8a. This commit
reintroduced the conditional that prevents the foreign keys from being
added to MySQL databases.

Fix test with `add_foreign_key` DSL

rename primary key sequence only if it exists

Always instrument ActiveSupport::Cache

Add always permitted parameters as a configurable option.

[Rafael Mendonça França + Gary S. Weaver]

The current approach is broken because it uses a thread local value
which means on multi-threaded environments it has to be turned on
per thread. Secondly, ActiveSupport::Notifications does not
instrument items when there are not subscribers so this flag is
unnecessary.

Update documentation for run_callbacks

In some cases run_callbacks will return nil.  I'm attempting to update the documentation for the method to clarify.

* General style fixes.
* Add changes to configuration guide.
* Add missing tests.

Leave the note for `ActiveModel`, since it can't yet detect mutations
(hopefully we can change this in time for 4.2). However, we now detect
mutations on all supported types in `ActiveRecord`, so we can note that
`_will_change!` is no longer needed there.

Closes #15942

Fix bug, when ':dependent => :destroy' violates foreign key constraints

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/associations/builder/association.rb
	activerecord/lib/active_record/associations/builder/has_one.rb

Detect in-place modifications on Strings