nebraska: Set noupdate status correctly.

I was setting noupdate in the wrong place. nebraska now responds
correctly to update requests for which no update is available.

BUG=chromium:892450
TEST=Attempt an update with no payloads more recent than the current
installed version on a test device.

Change-Id: If8c3bc9f2d68c69b6700c242f578ddeebd2e59fa
Reviewed-on: https://chromium-review.googlesource.com/1333935
Commit-Ready: Colin Howes <chowes@google.com>
Tested-by: Colin Howes <chowes@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/nebraska/nebraska.py b/nebraska/nebraska.py
index 72224e0..54dfeeb 100755
--- a/nebraska/nebraska.py
+++ b/nebraska/nebraska.py
@@ -71,7 +71,9 @@
     </updatecheck>
   """
 
-  NO_UPDATE = "noupdate"
+  UPDATE_CHECK_NO_UPDATE = """
+    <updatecheck status="noupdate"/>
+  """
 
   ERROR_NOT_FOUND = "error-unknownApplication"
 
@@ -409,7 +411,9 @@
         app_response.set('status', XMLResponseTemplates.ERROR_NOT_FOUND)
       elif self._app_request.request_type == \
           self._app_request.RequestType.UPDATE:
-        app_response.set('status', XMLResponseTemplates.NO_UPDATE)
+        app_response.set('status', "ok")
+        app_response.append(ElementTree.fromstring(
+            XMLResponseTemplates.UPDATE_CHECK_NO_UPDATE))
 
       return app_response