Return a write method from start_response, as required by the wsgi spec.

tornado/wsgi.py

@@ -211,11 +211,13 @@ class WSGIContainer(object):
 
     def __call__(self, request):
         data = {}
+        response = []
         def start_response(status, response_headers, exc_info=None):
             data["status"] = status
             data["headers"] = response_headers
-        response = self.wsgi_application(
-            WSGIContainer.environ(request), start_response)
+            return response.append
+        response.extend(self.wsgi_application(
+                WSGIContainer.environ(request), start_response))
         body = "".join(response)
         if hasattr(response, "close"):
             response.close()