LOCALHOST definition should match any 127.0.0.0/8 address

The entire 127.0.0.0/8 range is assigned to the loopback address, not
only 127.0.0.0/24. This patch allows ActionDispatch::Request::LOCALHOST
to match any IPv4 127.0.0.0/8 loopback address.

The only place that the #local? method was previously under test was
in the show_expectations_test.rb file. I don't particularly like that
that's implicitly where this code is under test, and I feel like I
should move some of that testing code into the
test/dispatch/request_test.rb file, but I wanted some feedback first.

Credit goes to @sriedel for discovering the issue and adding the
patch.
Add CHANGELOG for #14886

Also cleanup test a bit

[related #14886]
[related #14743]
Merge pull request #16221 from arthurnn/14886_pr

Cleanup + CHANGELOG for pr #14886