[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.
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]
Merge pull request #16300 from vast/mention-changes-to-nil-serialization-in-upgrading-from-3.2

Mention changes to `nil` handling in serialized attributes
update error message to reflect that the record could have been destroyed
Merge pull request #16456 from lsylvester/improve-active-record-error-message-on-update_columns-for-destroyed-records

Change error message in update_columns for destroyed objects
[ci skip] Fix broken url in plugins guide
Merge pull request #16472 from y-yagi/patch-1

[ci skip] Fix broken url in plugins guide
users_dont_suck_but_only_we_suck_and_only_our_tests_are_order_dependent!

Calling ActiveSupport::TestCase.i_suck_and_my_tests_are_order_dependent! in AS::TestCase makes
everyone's tests order dependent, which should never be done by the framework.