Fix instance variable not defined warning from Active Support test suite

Before

./Users/Juan/dev/rails/activesupport/test/file_update_checker_shared_tests.rb:20: warning: instance variable @tmpdir not initialized

After

No warnings

A small rewrite in a last attempt at writing obvious and portable code
without manual string manipulation.

Note that Pathname#== uses string comparison on Windows, so if client
code passes "C:\foo" and "c:/foo/bar" the predicate won't see the
former is an ascendant of the latter. Risky business.

Use the file watcher defined by the app config

This can make use of the FileEventedUpdateChecker, if available.

Whatever the inner loop selects, we already know is a descendant and
can be filtered out right away from dirs_sorted_by_nparts to skip
useless iterations.

Show middleware classes on /rails/info/properties

Closes #21230 by following the indication of @rafaelfranca:

> I think the output change would be simpler.
> What is really important to show is the class of the middleware, so we should change the output to show that.

Fix typo: "in" should be "is"

According to the W3 spec[1] the value should use a 1-based index
and not a 0-based index for the week number.

[1]: http://www.w3.org/TR/html-markup/datatypes.html#form.data.week

I spent a decent amount of time the other day trying to figure out why my active model including class wasn't generating the properly localized validation messages, and a note like this would have saved me a lot of time.

update docs for MessageEncryptor#new to recommend a KDF [ci skip]

fast and consistent return when local_cache does not exist

Add days_in_year method to Time class

Fix week_field returning invalid value

According to the W3 spec[1] the value should use a 1-based index
and not a 0-based index for the week number.

[1]: http://www.w3.org/TR/html-markup/datatypes.html#form.data.week