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]
Creating an SSE class to be used with ActionController::Live.
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
Fix a broken link
Merge pull request #11815 from yaotti/patch-1

Fix a broken link [ci skip]
Avoid Skip in test, have a unified test for order
Remove redundant `string_to_binary` from type-casting
Merge pull request #11820 from vipulnsward/remove_s2b

Remove redundant `string_to_binary` from type-casting
Make 'enable_extension' revertible

If 'enable_extension' is used in a migration's 'change' method, use
'disable_extension' on down migration (and vice-versa).
Add author_name field to post_params to the engines docs
Merge pull request #11830 from cr0t/master

Fixed small mistake in the engines docs
Execute conditional procs on controller filters only for current action.

:only and :except options for controller filters are now added before
:if and :unless. This prevents running :if and :unless procs when not
on the specified. Closes #11786.
Merge pull request #11799 from njakobsen/fix-controller-filter-callbacks

Execute conditional procs on controller filters only for current action
Merge pull request #11826 from etipton/enable_extensions_revertible

Make 'enable_extension' revertible