Return truthy value from head method

`QueueAdapters` now does lazy lookup

Update sprockets links to point to rails org

[ci skip] correct output of select

It was returning false in normal circumstances.
This broke the `head :ok and return if` construct.
Add appropriate test.

[CI SKIP] Correct output of options_for_select

Fix typo in the Testing Your Mailers docs

  * [ci skip]

Use request.session.id instead of request.session_options[:id]

Use a more conservative `const_get` over `constantize`

304 response should not include Content-Type header

As of the upgrade to Rack 1.5, request.session_options[:id] is no
longer populated. Reflect this change in the tests by using
request.session.id instead.

Related change in Rack:
https://github.com/rack/rack/commit/83a270d6

[ci skip] Take NOTE: as paragraph to make NOTE block

Isolate access to .default_scopes in ActiveRecord::Scoping::Default

`setup do` creates unnecessary allocations of proc objects in callbacks.
This prevents that from happening and results in faster code.

add `DateTime.now` to list of `TimeHelpers#travel_to` stubbing [ci skip]

Require pg ~> 0.18 to ensure Ruby 2.2 compatibility

Instead use .scope_attributes? consistently in ActiveRecord to check whether
there are attributes currently associated with the scope.

Move the implementation of .scope_attributes? and .scope_attributes to
ActiveRecord::Scoping because they don't particularly have to do specifically
with Named scopes and their only dependency, in the case of
.scope_attributes?, and only caller, in the case of .scope_attributes is
contained in Scoping.

ActiveJob: queue_adapter can be inherited

This allows different `queue_adapters` to be used in each `ActiveJob`
class heirarchy. Previously, all subclasses used a single global queue
adapter.

Versions of the pg gem earlier than 0.18.0 cannot be used safely with Ruby 2.2.
Specifically, pg 0.17 when used with Ruby 2.2 has a known bug that causes
random bits to be added to the end of strings. Further explanation here:
https://bitbucket.org/ged/ruby-pg/issue/210/crazy-bytes-being-added-to-record

Action Controller guide edits for grammar and clarity

`ActiveJob::QueueAdapters::*` are no longer singletons

This introduces undesirable `Rails.logger` formatters (such as the syslog
formatter) onto a `Logger.new(STDERR)` for the console. The production
logger may be going elsewhere than standard io, so we can't presume to
reuse its formatter.

With syslog, this causes missing newlines in the console, so irb prompts
start at the end of the last log message.

We can work to expose the console formatter in another way to address
the original issue.

This reverts commit 026ce5dd, reversing
changes made to 6f0a69c5.

Unnecessary usage of self in the guides [ci skip]

I deleted self from the callbacks guides code example because it's unnecessary and this way it's more like the other examples where self was not used.

[ci skip] Change to code element

This change was prompted by 598b8418.