Now the initial value of ActiveRecord.default_timezone is always :utc

`render :body` should just not set the `Content-Type` header. By
removing the header, it breaks the compatibility with other parts.

After this commit, `render :body` will returns `text/html` content type,
sets by default from `ActionDispatch::Response`, and it will preserve
the overridden content type if you override it.

Fixes #14197, #14238

This partially reverts commit 30473768.

This is a follow up to a668beff

use secrets.yml instead of secret_token.rb

Based on comments at
https://github.com/rails/docrails/commit/42417b0013819e573be9ffc5402e179003156e8e#commitcomment-5527251

[skip ci]

Fix a few typos. #present_in has been changed to #presence_in through
commit d61baee5.

[skip ci]

[skip ci]

Both examples given are incorrect, and no one can seem to come up with
a realistic situation where this is an actual problem. For now, it seems like
the best thing to do is just scrap the section altogether.

We noticed the rails gem increased in size between the beta and rc
versions, and the reason is that some guides images were updated but not
properly optimized, and one image changed from png to jpg, way bigger.

Converting the jpg image back to png, and using ImageOptim to optimize
all the images for web view, improves the situation [ci skip].

---

In current master/4-1-stable:

    508K  guides/assets/images/getting_started/

In v4.1.0.beta1:

    268K  guides/assets/images/getting_started/

In 4-0-stable

    328K  guides/assets/images/getting_started/

After the patch:

    260K  guides/assets/images/getting_started/

Remove 4-1 related entries from master [ci skip]

the fact that Rails is multi-thread by default is not very relevant. see https://github.com/rails/rails/pull/13998 [ci skip]

This change fixes the 42 instances of `end tag for "meta" omitted, but
OMITTAG NO was specified` error message that comes up when the full
railsguides are validated.

Full error list at
https://gist.github.com/prakashmurthy/9086679.

* Introduces `:plain`, `:html`, `:body` render option.
* Update guide to use `render :plain` instead of `render :text`.

[ci skip]

and upgrating guides

[ci skip]

Explain how form_for :article is able to pull in the properties of @article
Make it clear that article_id is generated due to the association set up

Add link to the rails function that uses X-Sendfile.
Add links to apache and nginx docs for the header