added authorization header to _post_multipart. File upload does not work without

This is just a simple fix for the issue described in this thread on ckan-dev: http://lists.okfn.org/pipermail/ckan-dev/2013-March/004387.html. 
File upload obviously needs some refactoring, there are still issues with submitting binary files and with generating the storage url. I'll try to look after that next week.