Remove caching_allowed? from ActionController::Caching

Where is this used? No other code references to this method and it
isn't being tested anywhere. No tests fail when commented out.
Make current_url? work with a HEAD method

ActionDispatch::Head was removed in favor of Rack::Head. But Rack::Head
does not convert GET requests to HEAD requests so we need to do
checking for HEAD requests ourselves.
Added a Changelog entry for fixing HEAD requests.
Merge pull request #9098 from firmhouse/add-head-requests

Make current_url? work with HEAD requests and remove caching_allowed? since it doesn't do anything