Remove warnings for already defined methods
Don't require BigDecimal serialization extension

Rails currently provides an extension to BigDecimal that redefines how
it is serialized to YAML. However, as noted in #12467, this does not
work as expected. When ActiveSupport is required, BigDecimal YAML
serialization does not maintain the object type. It instead ends up
serializing the number represented by the BigDecimal itself which, when
loaded by YAML later, becomes a Float:

```ruby
require 'yaml'
require 'bigdecimal'

yaml = BigDecimal('13.37').to_yaml
YAML.load(yaml).class

require 'active_support/all'

yaml = BigDecimal('13.37').to_yaml
YAML.load(yaml).class
```

@tenderlove posits that we should deprecate the custom BigDecimal
serialization and let Ruby handle it. For the time being, users who
require this serialization for backwards compatibility can manually
`require 'active_support/core_ext/big_decimal/yaml_conversions'`.

This will close #12467 and deprecate the custom BigDecimal#to_yaml.

Signed-off-by: David Celis <me@davidcel.is>
Remove BigDecimal#to_d

This was backported for Ruby 1.8 support and is no longer needed.

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #13911 from davidcelis/remove-bigdecimal-serialization

Deprecate custom BigDecimal serialization

Conflicts:
	activesupport/CHANGELOG.md
Fix isolated tests
Assert the file is deprecated
Adding an documentation example and a test to button_to with path

I did not see in the docs that `button_to` supports not only URLs but paths as well. I documented this functionality with a unit tests and added an example to the docs as well.
Merge pull request #13924 from adomokos/adding_path_examples_to_button_to_tests

Adding a documentation example and a test to button_to with path
fix HABTM w/out primary key errors on destruction
Fixes the camelCase variable name in the docs [ci skip]
Merge pull request #13926 from fredwu/doc_polymorphic_sti_var_name_typo

Fixes the camelCase variable name in the docs [ci skip]
Fixed minor typo [ci skip]
Merge pull request #13930 from calvintam/master

Fixed minor typo [ci skip]
`rails_guides/helpers.rb` requires `YAML` to run.

Resolve the following error when running `bundle exec rake guides:generate:html`:

```
/Users/senny/.rbenv/versions/2.0.0-p353/bin/ruby rails_guides.rb
Generating 2_2_release_notes.md as 2_2_release_notes.html
/Users/senny/Projects/rails/guides/rails_guides/helpers.rb:17:in `documents_by_section': uninitialized constant RailsGuides::Helpers::YAML (ActionView::Template::Error)
	from /Users/senny/Projects/rails/guides/rails_guides/helpers.rb:32:in `docs_for_menu'
	from /Users/senny/Projects/rails/guides/source/layout.html.erb:56:in `block in ___sers_senny__rojects_rails_guides_source_layout_html_erb___3094858039481335962_70118647133480'
	from /Users/senny/Projects/rails/guides/source/layout.html.erb:54:in `each'
	from /Users/senny/Projects/rails/guides/source/layout.html.erb:54:in `___sers_senny__rojects_rails_guides_source_layout_html_erb___3094858039481335962_70118647133480'
	from /Users/senny/Projects/rails/actionview/lib/action_view/template.rb:143:in `block in render'
	from /Users/senny/Projects/rails/activesupport/lib/active_support/notifications.rb:161:in `instrument'
	from /Users/senny/Projects/rails/actionview/lib/action_view/template.rb:337:in `instrument'
	from /Users/senny/Projects/rails/actionview/lib/action_view/template.rb:141:in `render'
	from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/template_renderer.rb:61:in `render_with_layout'
	from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/template_renderer.rb:47:in `render_template'
	from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/template_renderer.rb:17:in `render'
	from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/renderer.rb:42:in `render_template'
	from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/renderer.rb:23:in `render'
	from /Users/senny/Projects/rails/actionview/lib/action_view/helpers/rendering_helper.rb:24:in `render'
	from /Users/senny/Projects/rails/guides/rails_guides/markdown.rb:160:in `render_page'
	from /Users/senny/Projects/rails/guides/rails_guides/markdown.rb:25:in `render'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:205:in `block in generate_guide'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:195:in `open'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:195:in `generate_guide'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:144:in `block in generate_guides'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:142:in `each'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:142:in `generate_guides'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:97:in `generate'
	from rails_guides.rb:63:in `<main>'
rake aborted!
Command failed with status (1): [/Users/senny/.rbenv/versions/2.0.0-p353/bi...]
/Users/senny/Projects/rails/guides/Rakefile:11:in `block (3 levels) in <top (required)>'
Tasks: TOP => guides:generate:html
(See full trace by running task with --trace)
```
docs, reference to ruby-lang.org. refs #13492. [ci skip]
docs, revisit polymorphic associations with STI example. [ci skip]

This is a follow up to #13926.

/cc @fxn
pass `habtm :autosave` to underlying `hm:t` association. Closes #13923.
some wording format changes. [ci skip]
docs, be clear that `options` is a hash. Closes #11904. [ci skip].
Merge pull request #11770 from timruffles/doc_ajax_xhr

be more specific about csrf token and ajax - not whitelisted outside of jquery-rails [ci skip]