Corrected test and multipart size check

test_multipart_upload was failing because the .should.throw methods expects a function to call that will raise the exception. Just removing the ()'s passes the complete_upload() function rather than calling it directly.

Also, the size check on multipart uploads does not apply to the total file size, but applies to each individual part except for the last part.

Hopefully this can now be merged in to 'spulec's repository.