Prevent warnings when SWORD statement provided as an Atom feed

Minor edits to prevent warnings when creating SWORDAppStatement objects from SWORD statements returned by the [Dataverse Data Deposit API](http://thedata.harvard.edu/guides/dataverse-api-main.html#display-a-study-statement) as Atom feeds.

(warnings reported by OJS test sites for the [Dataverse plugin](http://projects.iq.harvard.edu/ojs-dvn/home))