Add Finder API changes to upgrading guide [ci skip]

Adds Akshay Surve to Rails Guides credits list [ci skip]

Remove extra case.

Add error message for including nokogiri in Gemfile for rake doc:guides

Remove dependency on Action View from Action Pack

This set of changes removes the need of using Action View with Action Pack.
Now you may use controllers without Action View, by rendering `:text` response
or alternatively you can plug in your own rendering logic. This is especially
handy when you're just dealing with APIs and don't need to include entire
Action View just to render simple JSON responses.

Closes #11899.

Changing deprecation_horizon to be Rails 4.2

Removing instances of string class_names in fixtures.

Make it oneliner

We're not dealing with templates here

[ci skip]

The ones that were actually testing AV functionality and should belong in there

[ci skip]

HTML Scanner is part of ActionView and it should be loaded along with it

All rendering test remain in AV. Head, ETag etc. test stay in AP

We don't need them. They'll be loaded automaticly if AV will be included

It's needed for tests

Move that part to AV railtie