Bug fix in serializing to xml or json where both :only and :except are passed 

There was a bug in determining if the uploader should be serialized
when both :only and :except were passed. 

The uploader was being serialized even if it didn't appear in the :only option, because it also didn't appear in the :except option.

:only takes precedence. :except should be taken into account only if :only isn't passed

This bug was especially important with ActiveRecord's to_xml
implementation, which adds an additional :except => :type to the
options hash. This revealed the bug even when I was only passing :only

Please note that this bug may also be present in carrierwave's version for other ORMs, like mongoid, sequel, etc.