Add an after_bundle callback in Rails plugin templates

Add additional documentation on Headers#[] [ci skip]

The detach used by close! seems to be making EM very sad on Travis.

Issue #16519 covers confusion potentially caused by how HTTP
headers, that contain underscores in their names, are retrieved
through `ActionDispatch::Http::Headers#[]`.

This confusion has its origin in how a CGI maps HTTP header names
to variable names. Even though underscores in header names
are rarely encountered, they are valid according to RFC822 [1].
Nonetheless CGI like variable names, as requested by the Rack
specfication, will only contain underscores and therefore the
original header name cannot be recovered after the Rack server passed
on the environemnt hash. Please, see also the disscussion on
StackOverflow [2], which also links to an explaination in the
nginx documentation [3].

[1] http://www.ietf.org/rfc/rfc822.txt
[2] http://stackoverflow.com/questions/22856136/why-underscores-are-forbidden-in-http-header-names
[3] https://www.nginx.com/resources/wiki/start/topics/tutorials/config_pitfalls/#missing-disappearing-http-headers

It's not strictly necessary, and maybe this will help with the current
test failure.

remove require logging

EventMachine -> concurrent-ruby, take two

We may still try to send to it.

Also, address the possibility of the listen thread dying and needing to
be respawned. As a bonus, we now defer construction of the thread until
we are first given something to monitor.

They're all at risk of races on the first requests.

These methods are more expensive than the alternatives, and have strange semantics that are likely undesirable.

Fix broken number_to_currency conversion tests

Remove `limit: 11` as backward-compatibility with Rails 2.0

Clarify DatabaseStatements#execute docs re: memory usage.

Change number_to_currency behavior for checking negativity of a number

- Instead of using `to_f.phase`, just use `to_f.negative`?.
- This change works same for all cases except when number is "-0.0".
      -0.0.to_f.negative? => false
      -0.0.to_f.phase? => pi
- So -0.0 will be treated as positive from now onwards.
- So this change reverts changes from https://github.com/rails/rails/pull/6512.
- But it should be acceptable as we could not find any currency which
  supports negative zeros.

Extract options for cache_fragment_name method to allow expires_in on cache method

Restore ability to pass extra options to cache stores

Getting Started Guide: Update Ruby version to 2.3.0p0 [ci skip]

Docs review of api_app.md

Accept header is taken from what Safari on El Capitan sends:

```ruby
require 'benchmark/ips'
require 'action_dispatch/http/mime_type'
require 'active_support/all'

accept = 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8'

Benchmark.ips do |x|
  x.report "omg" do
    Mime::Type.parse(accept)
  end
end
```

Before:

```
[aaron@TC actionpack (master)]$ be ruby ../x.rb
Calculating -------------------------------------
                 omg     3.181k i/100ms
-------------------------------------------------
                 omg     35.062k (±12.8%) i/s -    174.955k
[aaron@TC actionpack (master)]$ be ruby ../x.rb
Calculating -------------------------------------
                 omg     3.153k i/100ms
-------------------------------------------------
                 omg     33.724k (±12.4%) i/s -    167.109k
[aaron@TC actionpack (master)]$ be ruby ../x.rb
Calculating -------------------------------------
                 omg     3.575k i/100ms
-------------------------------------------------
                 omg     37.251k (±10.4%) i/s -    185.900k
```

After:

```
[aaron@TC actionpack (master)]$ be ruby ../x.rb
Calculating -------------------------------------
                 omg     3.365k i/100ms
-------------------------------------------------
                 omg     40.069k (±16.1%) i/s -    198.535k
[aaron@TC actionpack (master)]$ be ruby ../x.rb
Calculating -------------------------------------
                 omg     4.168k i/100ms
-------------------------------------------------
                 omg     47.596k (± 7.7%) i/s -    237.576k
[aaron@TC actionpack (master)]$ be ruby ../x.rb
Calculating -------------------------------------
                 omg     4.282k i/100ms
-------------------------------------------------
                 omg     43.626k (±17.7%) i/s -    209.818k
```

Pass through correcting api_app.md. The list of included modules and
middleware was tested through a sample API app, and was listed in the
same order an end user would see in their terminal.

[ci skip]

It is not always there anymore

[ci skip]

[ci skip]

The documentation team didn't reviewed it yet so it can't be published.

we never use this custom array outside the mime type `parse` method.  We
can reduce the interaction to just a regular array, so we should use
that instead (IOW, there was nothing special about AcceptList so we
should remove it).

Put "Using Rails for API-only Applications" in table of contents

Remove empty lines in Rails development logger

When I was looking for more info regarding this the only way I ended up
on that page was by googling something along the lines of "rails new
api" (as I wanted to find out what are the proper parameters when
generating api app). I think it's beneficial to have that page in table
of contents.

This works on OSX but for some reason travis is throwing a
```
  1) Error:
ExpiresInRenderTest#test_dynamic_render_with_absolute_path:
NoMethodError: undefined method `unlink' for nil:NilClass
```
Looking at other tests in Railties the file has a name and we close
it before unlinking, so I'm going to try that.