[ci skip] Mention the change in nil handling for serialized attributes
in "Upgrading from Rails 3.2 to Rails 4.0"
[ci skip] Clarify the "Serialized Attributes" section

Mention that the change in nil handling for serialized attributes
affects custom coders.
Add config option for cookies digest

You can now configure custom digest for cookies in the same way as `serializer`:

config.action_dispatch.cookies_digest = \SHA256'
Return preconfigured AS::MessageVerifier
Merge pull request #16434 from strzalek/cookies-digest-config-option

Add digest config option for SignedCookiesJar
Revert "Merge pull request #16434 from strzalek/cookies-digest-config-option"

This reverts commit 705977620539e2be6548027042f33175ebdc2505, reversing
changes made to dde91e9bf5ab246f0f684b40288b272f4ba9a699.

IT BROKE THE BUILD!!!
Docs: grammar fix for fetch. [ci skip]
Merge pull request #16437 from mscoutermarsh/master

Docs: grammar fix for fetch. [ci skip]
Using delegated connection method
Pass block for logging.

This follows the good practice listed on http://guides.rubyonrails.org/debugging_rails_applications.html#impact-of-logs-on-performance.
[ci skip] Changed options to +options+ for rdoc
Update README.md

Ruby on Rails Tutorial link updated.
Merge pull request #16443 from duccioarmenise/patch-1

Rails Tutorial link updated
Merge pull request #16439 from voxxit/patch-1

Using delegated connection method
Merge pull request #16440 from tgxworld/pass_log_in_block

Pass block for logging.
Merge pull request #16441 from tomkadwill/activerecord_associations_options_param

[ci skip] Changed options to +options+ for rdoc
[ci skip] Fix sample code in engines guide
Merge pull request #16451 from y-yagi/patch-2

[ci skip] Fix sample code in engines guide
Cleanup Guides Guidelines [ci skip]

* Copy editing for clarity in several locations
* Rename "Titles" section to "Headings"
* Fix incorrect anchor in link to API Documentation Guidelines for filenames
Merge pull request #16453 from reshleman/re-cleanup-guides-guidelines

Cleanup Guides Guidelines [ci skip]