Improve the testing guide [ci skip]

Improve the "Testing Rails Applications" guide adding a section for
testing helpers and a little note about associations with ActiveRecord
and YAML fixtures.

Also add a link to Fixture Builder ; yet another replacement for
fixtures such as Machinist or FactoryGirl.
Added SanitizeHelper to rails guide docs [ci skip]

Added SanitizeHelper to rails guide docs [ci skip]

Added SanitizeHelper to rails guide docs update [ci skip]
Added Action View in 'rake about' [ci skip]
added String#to_date documentation to Rails 4 upgrade guide

[ci skip]
Removing obsolete html tags in favor of markdown code block.
Revert "Removing obsolete html tags in favor of markdown code block"

This reverts commit 157dc275da0ad38635337b7c1d96de656d91de8e.
Minor align BR tags with HTML void - with reference to the generators code change - https://github.com/rails/rails/commit/f038d4cc5fe1ac21f92c7e32c61ff85a0c816f3f
Add tests for ActiveModel::Serializers::JSON#as_json ordering
Avoid Skip in test, have a unified test for order
Upgrade ActiveRecord-JDBC-Adapter to master branch

Switch to master branch which provides Version 1.3.x of AR-JDBC adapter.
It strives to provide ActiveRecord 4.x compatibility (as well as still supporting 2.3 and 3.x) from a single code base. It's a recommended update for all AR-JDBC 1.2.x users.
drop extra variable
Merge pull request #11846 from gaurish/jdbc-upgrade

Upgrade ActiveRecord-JDBC-Adapter to v1.3 for better compatibility with ActiveRecord 4
Fix interactions between :before_add callbacks and nested attributes assignment

Issue #1: :before_add callback is called when nested attributes assignment assigns to existing record if the association is not yet loaded
Issue #2: Nested Attributes assignment does not affect the record in the association target when callback triggers loading of the association
Restore the use of `#add_to_target` for nested attribute updates on existing records, and don't bother updating the association if the update is going to be rejected anyway.

This requires adding a `skip_callbacks` argument to `#add_to_target`
so that we don't call the callbacks multiple times in this case,
which is functionally an application of existing association data,
rather than an addition of a new record to the association.
Refactor NestedAttributesWithCallbacksTest for clarity

1) Use `assert` and `refute` where possible.
2) Separately include the setup, subject, and assertions in each test - don't hide the tested call in an assertion method.
3) Name things based on their role rather than incidental facts about them - e.g. `@bird[1]` -> `bird_to_destroy`. `bird2_deletion_attributes` -> `destroy_bird_attributes`.
4) Use more succinct naming where possible - e.g. `birds_with_callback` -> `birds_with_add`, `@pirate_with_two_birds` -> `@pirate`
Merge pull request #11525 from Empact/nested_attributes_with_callbacks_bug

Improve #5476 - "Nested attributes with callbacks bugfix" to use add_to_target and clearer tests
Use assert_not rather than refute
Merge pull request #11838 from vipulnsward/drop_extra_var

drop extra variable
Added comment to a guide about necessity of `:dependent` option for associations with NOT NULL constraint
Merge pull request #11852 from prijutme4ty/guide_addition

Added comment to a guide about necessity of `:dependent` option for asso... [ci skip]