TLDR: always return an object that responds to the query methods from
request.format, and do not touch Mime::Type[] lookup to avoid bugs.

---

Long version:

The initial issue was about being able to do checks like
request.format.html? for request with an unknown format, where
request.format would be nil.

This is where the issue came from at first in #7837 and #8085
(merged in cba05887), but the
implementation went down the path of adding this to the mime type
lookup logic.

This unfortunately introduced subtle bugs, for instance in the merged
commit a test related to send_file had to be changed to accomodate the
introduction of the NullType.

Later another bug was found in #13064, related to the content-type being
shown as #<Mime::NullType:...> for templates with localized extensions
but no format included. This one was fixed in #13133, merged in
43962d6e.

Besides that, custom handlers were not receiving the proper template
formats anymore when passing through the rendering process, because of
the NullType addition. That was found while migrating an application
from 3.2 to 4.0 that uses the Markerb gem (a custom handler that
generates both text and html emails from a markdown template).

---

This changes the implementation moving away from returning this null
object from the mime lookup, and still fixes the initial issue where
request.format.zomg? would raise an exception for unknown formats due to
request.format being nil.

This PR fixes #13064 regression bug introduced by the #8085

Now in _process_format when the format is a Mime::NullType nothing is written in self.content_type.
In this way the method Response#assign_default_content_type_and_charset can
write the the default mime_type.

Make it oneliner

We're not dealing with templates here

[ci skip]

[ci skip]

This reverts commit f4d602aff6cec80304b131ecfcc2676d0304f0cc.

If a request has unknown format (eg. /foo.bar), the renderer
fallbacks to default format.

This patch reverts Rails 3.2 behavior after c2267db3 commit.

Fixes issue #9654.

Closes #5025 part 2

This seems to be required only when calling render :partial with an
empty collection from a controller. This call happens to return no
content, letting the response body empty, which means to Rails that it
should go on and try to find a template to render based on the current
action name, thus failing hard.

Although tests keep all green, we need to check a better way to fix
this.

Options :nothing and :text => nil should be handled by
ActionController::Rendering instead.

Finish cleaning up rendering stack from views and move assigns evaluation to controller (so plugins and/or controllers can overwrite just one method).

Change AV formats so they can delegate to the controller. Now users (or plugins) can override details_for_render in their controllers and add appropriate additional details. Now if only they could *do* something with those details...