Fixed typo one line in layouts and rendering: stylesheet_link_tag for file in subdirectory.

* Fix one error
* Improve wording
* Improve punctuation

[ci skip]

Rack::Lock actually sets rack.multithread to `false`, not `true`.

Presumably because once inside the mutex, the app can function as if it were single threaded.

Removed first slash. The subdirectory will nto be found with slash befor it.

Update documentation for yml usage against https://github.com/rails/rails/issues/8612

This reverts commit a3309caf.

Reason: docrails isn't the place to do this.

[ci skip]

This reverts commit 0d8ca812.

Reason: docrails isn't the place to do this. Also, I feel this is
not really required.

[ci skip]

This reverts commit 41381c19.

Reason: This doesn't quite fit the flow where it's added. Let's add this
someplace else if it's explicitly required.

[ci skip]

Remove 'assigned but unused variable' warning

rewrite order dependent test case

As reported (https://github.com/rails/rails/pull/8185#issuecomment-11702226)
this test relied on the order a hash was serialized. Comparing the parsed
hash makes the test no longer order dependent.

Ruby 1.9 style hash in Route Inspector

Tagging every message in tests makes the logs really wide. It's great
for grepping, but annoying to open in an editor or a narrow terminal.
Try out a different approach: spit out a heading before each test.

collection_check_boxes to a private method.

It will make easier to extend.

Example:

    class CollectionRadioButtons < ActionView::Helpers::Tags::CollectionRadioButtons
      include CollectionExtensions

      def render
        wrap_rendered_collection(super, @options)
      end

      private

        def render_component(builder)
          builder.radio_button + builder.label(:class => "collection_radio_buttons")
        end
    end

`false`

These tips were documenting an inconsistency issue https://github.com/rails/rails/issues/8621. That issue is resolved by https://github.com/rails/rails/issues/8622.

[ci skip]

refactor highlight method: return fast if text or phrases are blank

right place.

The EachValidator#validate already handle :allow_blank and :allow_nil,
correctly.

Closes #8622.

Fix #8621.

Conflicts:
	activemodel/lib/active_model/errors.rb

We should mention that the ordering matters with regards
to timestamps, and not imply that it's just for uniqueness
purposes.

Closes #8610.

quote column names in generated fixture files

Validate :on option on after_commit and after_rollback callbacks

is false.

If it is nil we can't raise the exception

more descriptive return parameters