Response data missing in Spring MVC test

I have a controller that writes to the ModelMap like this:

@Controller
@RequestMapping("/dataset")
public class DatasetController {

    @Autowired
    DatasetDao datasetDao;

    @RequestMapping(method = RequestMethod.GET)
    public String getDataset(@RequestParam String name, ModelMap model) {
        Dataset ds = datasetDao.get(name);
        model.addAttribute("response", new DatasetResponse(ds));
        return "dataset/create";
    }

I want to write a test case that gets a dataset and then performs more actions based on its contents. But, I'm having trouble getting data out of the service. My test case so far is:

@Test
public void testProducesXml() throws Exception {
    mockMvc.perform(get("/dataset.xml").param("name", "foo"))
        .andDo(print())
        .andExpect(status().isOk())
        .andExpect(model().attribute("dataset",
            hasProperty("name", is("foo"))))
        .andExpect(xpath("/dataset/name").string("foo"));
}

The model().attribute line passes, but the test fails at xpath with:

org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; Premature end of file.

Ultimately I'd like to get a copy of the DatasetResponse in my test function so I can do other things with it. I tried using andReturn(), but that fails for the same reason that xpath fails.

It turns out that even though there's a DatasetResponse in the model, the body of the response is empty:

ModelAndView:
    View name = dataset/create
         View = null
    Attribute = response
        value = org.vpac.web.model.response.DatasetResponse@457fdd58
       errors = []

MockHttpServletResponse:
       Status = 200
Error message = null
      Headers = {}
 Content type = null
         Body = 

However if I start the app in Tomcat and go to /dataset.xml?name=foo in my browser, I can see the data as XML.

So, how can I get a reference to the DatasetResponse?