fails:File.world_readable? returns nil if the file is chmod 600
fails:File.world_readable? returns nil if the file is chmod 000
fails:File.world_readable? returns nil if the file is chmod 700
fails:File.world_readable? returns a Fixnum if the file is chmod 644
fails:File.world_readable? returns a Fixnum if the file is a directory and chmod 644
fails:File.world_readable? coerces the argument with #to_path
fails:File.world_readable? returns nil if the file does not exist
