As of Ruby 2.2, Dir.glob returns matches with the correct case as opposed
to in previous versions where the matches were returned with the case of
the pattern. In certain scenarios on Mac OS X the app can be booted with
the incorrect case, e.g. incorrect case on Pow symlink). This is because
the app is built by Rack::Builder evaluating a string from the config.ru
file and passing the incorrectly cased path to the eval method.

As Dir.glob is the only method in Ruby 2.2 that appears to do this case
correction currently we need to rewrite the find_root_with_flag method
to use it so that it corrects the case for when the applications paths
instance is created.

Fixes #18660.