Fix bug in reprocess method.

Fixed a bug in the attachment reprocess method that occurs if a default
style other than 'original' is set.
Refactor tests back into green.

Refactored tests back into green since making latest changes and bug
fixes.
Make instanceWrite method public.

The instanceWrite method has now been made public.  This method provides
a convenient mechanism to set properties (file_size, content_type, etc)
for an attachment on a model.