Generate claim XML

Payment and UserCharacteristics elements are inserted to make the XML validate against the schema but are left blank for now.


`instance_double`'s only work when you set expectations explicitly. The short syntax I used doesn't help. I had planned to complement the isolated test with a full integration test but put that off to a further story (when we implement the submitted page). This would have caught it. Maybe it's better just to use real models or perhaps FactoryGirl.