Fixed keyError in download-fixtures.

Fixes [fb-case](http://manage.dimagi.com/default.asp?pg=pgEditBug&ixBug=67355&command=reply&ixBugEventLatest=384106&ixBugEvent=384097#384329). The problem occurs because 'fields' attribute of data-item is not updated when the same is edited for corresponding data-type using website.