Clear inflections after test.

[ci skip] Improve form_helpers.md guide.

Fix documentation

To prevent `ArgumentError`, `config.action_view.sanitized_allowed_attributes` should be assigned by an array instead of a list.

```
/Users/tonytonyjan/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/set.rb:98:in `do_with_enum': value must be enumerable (ArgumentError)
	from /Users/tonytonyjan/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/set.rb:355:in `merge'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/actionview-4.1.1/lib/action_view/helpers/sanitize_helper.rb:211:in `sanitized_allowed_attributes='
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/actionview-4.1.1/lib/action_view/railtie.rb:26:in `block (3 levels) in <class:Railtie>'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/actionview-4.1.1/lib/action_view/railtie.rb:25:in `each'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/actionview-4.1.1/lib/action_view/railtie.rb:25:in `block (2 levels) in <class:Railtie>'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/activesupport-4.1.1/lib/active_support/lazy_load_hooks.rb:38:in `instance_eval'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/activesupport-4.1.1/lib/active_support/lazy_load_hooks.rb:38:in `execute_hook'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/activesupport-4.1.1/lib/active_support/lazy_load_hooks.rb:28:in `block in on_load'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/activesupport-4.1.1/lib/active_support/lazy_load_hooks.rb:27:in `each'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/activesupport-4.1.1/lib/active_support/lazy_load_hooks.rb:27:in `on_load'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/actionview-4.1.1/lib/action_view/railtie.rb:24:in `block in <class:Railtie>'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/initializable.rb:30:in `instance_exec'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/initializable.rb:30:in `run'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/initializable.rb:55:in `block in run_initializers'
	from /Users/tonytonyjan/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/tsort.rb:226:in `block in tsort_each'
	from /Users/tonytonyjan/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/tsort.rb:348:in `block (2 levels) in each_strongly_connected_component'
	from /Users/tonytonyjan/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/tsort.rb:427:in `each_strongly_connected_component_from'
	from /Users/tonytonyjan/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/tsort.rb:347:in `block in each_strongly_connected_component'
	from /Users/tonytonyjan/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/tsort.rb:345:in `each'
	from /Users/tonytonyjan/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/tsort.rb:345:in `call'
	from /Users/tonytonyjan/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/tsort.rb:345:in `each_strongly_connected_component'
	from /Users/tonytonyjan/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/tsort.rb:224:in `tsort_each'
	from /Users/tonytonyjan/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/tsort.rb:205:in `tsort_each'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/initializable.rb:54:in `run_initializers'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/application.rb:288:in `initialize!'
	from /Users/tonytonyjan/Dropbox/home/codes/penta_ruby/config/environment.rb:5:in `<top (required)>'
	from /Users/tonytonyjan/Dropbox/home/codes/penta_ruby/config.ru:3:in `require'
	from /Users/tonytonyjan/Dropbox/home/codes/penta_ruby/config.ru:3:in `block in <main>'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/rack-1.5.2/lib/rack/builder.rb:55:in `instance_eval'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/rack-1.5.2/lib/rack/builder.rb:55:in `initialize'
	from /Users/tonytonyjan/Dropbox/home/codes/penta_ruby/config.ru:in `new'
	from /Users/tonytonyjan/Dropbox/home/codes/penta_ruby/config.ru:in `<main>'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/rack-1.5.2/lib/rack/builder.rb:49:in `eval'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/rack-1.5.2/lib/rack/builder.rb:49:in `new_from_string'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/rack-1.5.2/lib/rack/builder.rb:40:in `parse_file'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/rack-1.5.2/lib/rack/server.rb:277:in `build_app_and_options_from_config'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/rack-1.5.2/lib/rack/server.rb:199:in `app'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/commands/server.rb:50:in `app'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/rack-1.5.2/lib/rack/server.rb:314:in `wrapped_app'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/commands/server.rb:130:in `log_to_stdout'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/commands/server.rb:67:in `start'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/commands/commands_tasks.rb:81:in `block in server'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/commands/commands_tasks.rb:76:in `tap'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/commands/commands_tasks.rb:76:in `server'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/commands/commands_tasks.rb:40:in `run_command!'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/railties-4.1.1/lib/rails/commands.rb:17:in `<top (required)>'
	from /Users/tonytonyjan/Dropbox/home/codes/penta_ruby/bin/rails:8:in `require'
	from /Users/tonytonyjan/Dropbox/home/codes/penta_ruby/bin/rails:8:in `<top (required)>'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/spring-1.1.3/lib/spring/client/rails.rb:27:in `load'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/spring-1.1.3/lib/spring/client/rails.rb:27:in `call'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/spring-1.1.3/lib/spring/client/command.rb:7:in `call'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/spring-1.1.3/lib/spring/client.rb:26:in `run'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/spring-1.1.3/bin/spring:48:in `<top (required)>'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/spring-1.1.3/lib/spring/binstub.rb:11:in `load'
	from /Users/tonytonyjan/.rvm/gems/ruby-2.1.2/gems/spring-1.1.3/lib/spring/binstub.rb:11:in `<top (required)>'
	from /Users/tonytonyjan/Dropbox/home/codes/penta_ruby/bin/spring:16:in `require'
	from /Users/tonytonyjan/Dropbox/home/codes/penta_ruby/bin/spring:16:in `<top (required)>'
	from bin/rails:3:in `load'
	from bin/rails:3:in `<main>'
```

Keep  closer to other methods that touch @transaction

Maintain column order when overriding existing columns

Working towards re-implementing serialized attributes to use the
properties API exposed the need for this, as serializing a column
shouldn't change the order of the columns.

Ensure custom properties work correctly with inheritance

Move types to the top level `ActiveRecord` namespace

Initial doc for TransactionIsolationError [ci skip]

This reverts commit ec0a2b57, reversing
changes made to a8bd7b16.

This reverts commit 555ec365

This reverts commit 555ec365

By default, Rails uses the `default` Rake task to run all tests.
This commit changed the environment of the default task to `development`.
This clears the development database and has other negative consequences.

Avoid hardcoded `request_forgery_protection_token` value in teardown.

Remove unused `Column#coder`

Unregister Mime::Type in teardown.

It appears this property was added, but never actually used. It would be
broken if it were, as it only type casts one way.

Add and remove renderer inside the test to prevent leak.

Move I18n.locale setting into setup and teardown.

Add with_default_charset helper.

The solution presented in this patch is not efficient. We should replace it
in the near future. The following needs to be worked out:
  * Is `@attributes` storing the Ruby or SQL representation?
  * `cacheable_column?` is broken but `hstore` and `json` rely on that behavior

Refs #15369.

/cc @sgrif @rafaelfranca

`ActiveRecord::ConnectionAdapters::Type::Value` =>
`ActiveRecord::Type::Value`

Remove AR Properties from the public API

Making this part of the public API was premature, let's make it private
again while I continue to work on the surrounding code.

rake railties:install:migrations respects the order of railties

Add test case for clear mappings

This is the continuation of the work started at
9d62e048