Refactors ActiveFedora::File#size to use two new methods, ActiveFedora::File#persisted_size and ActiveFedora::File#dirty_size.
This fixes #560 where has_content? was returning false when there was unpersisted content, b/c it was only checking the size of persisted content.
Merge pull request #567 from afred/560-refactor-size-methods

Refactors ActiveFedora::File#size to use two new methods, ActiveFedora::...