`Update` method doesn't work properly for custom attribute

[yahortsaryk]

Hi, thanks for your work, this is great package. I've found an annoying bug:

storage.update(id, fnCallback, [change]); method updates custom attribute for file metadata only, but doesn't update it for config file inside storage directory, which makes it impossible to search (find) by custom attribute. Steps to reproduce:

• Insert new file without custom attribute
• Call storage.update(id, fnCallback, [change]); for this file

Result: I see new attribute in /000-000-001/000000005.data file but don't see it in /000-000-005/config