Fixed incorrect path generation for sprites in Rails 4

[dmeremyanin]

Before:

if subdirectory.nil?
  bust_cache_path = generated_images_path
else
  bust_cache_path = generated_images_path + "/" + subdirectory
end

subdirectory will never be nil? there.
It will be . in case path is icons-sXXXXXXXXXX.png and bust_cache_path becomes app/assets/images/.

Moving the sprited images into a deeper subfolder works around the problem, subdirectory becomes folder_name instead of ..

Fixes #94, #162, #187, #188

[dmeremyanin]

@scottdavis could you take a look please?

[salimane]

+1 @scottdavis

[braindeaf]

I'm seeing signs that the cache busting no longer works. I'm stuck in Rails 4 upgrade hell with this particular issue going on for a week now. It appears that the sprockets_entries for those keys don't get deleted, even though that's crazy because they're just two hashes.

      entry = generated_images_dir.join(dir).to_s
      sprockets_entries.delete(entry)

      stat = generated_images_dir.join(path).to_s
      sprockets_stats.delete(stat)

I'm using compass 1.0.3, compass-rails 3.0.2, Rails 4.0.13, sprockets 3.7.0, sprockets-rails 2.3.3. Any thoughts?