use define_singleton_method instead of class_eval

Add :nodoc: mark to PerThreadRegistry#method_missing [ci skip]

Update github links

Existing code was delegating to the instance with delegate
macro calls, or invoking the instance method to reach
the object and call its instance methods.

But the point is to have a clean class-level interface where
the thread local instance is hidden in the implementation.

References #11c69738.
References #10198.

remove unused variable

Registries have class-level accessors to write clean code, let's
use them. This makes style uniform also with existing usage in
ScopeRegistry and InstrumentationRegistry.

If performance of the method_missing callback was ever considered to
be a concern, then we should stop using it altogether and probably
remove the callback. But while we have the feature we should use it.

The previous version was kind of duplicating the documentation of
AS::PerThreadRegistry. Just say how to use it, the thread locals
registry is know part of our vocabulary (though a pointer to
Active Support is added for reference).

* It focuses on how to use it.

* Removes some ambigueties in the original docs about whether the state is stored in the class.

* Documents it provides class-level accessors via method_missing.

* Documents that if the extended class has an initializer, it must accept no arguments.

This reverts commit 81f24337, reversing
changes made to c7673b09.

Extract method declaration regexp for assert_method into a constant

Added tests for `eager_load` config option rake tasks

Added tests for `eager_load` config option to do not eager load for rake tasks if `eager_load` is `true`

align the result of expressions vertically [ci skip].

Add Chatham Islands Time (UTC+1245) support.

Also update the documentation on the number of supported time zones.

Use unified and clean formatting in CHANGELOGS. [ci skip]

See https://github.com/lifo/docrails/commit/edeb98c2745cbf29725eca87332cefb4f273986a#commitcomment-2995521

Simpler, thanks to @rubys for the hint.

refactor order hash test; remove unused method param

Conflicts:
	guides/source/action_mailer_basics.md

Unabbreviate AP to ActionPack [ci skip]

Revert "added details to section 4.1.2 of the Asset Pipeline guide, describing how to configure Apache to serve the gzipped version of the precompiled assets"

This reverts commit aec466e7.

Reason: We're not sure about the correctness and won't want to document
something like this with doubts.