devserver: IsDelta -> IsDeltaPayload; Boolean value in lowercase

This fixes two problems: (i) the devserver uses 'IsDelta' as the field
name for the Omaha response the reflects whether a payload is a delta,
which is wrong (should be 'IsDeltaPayload');  (ii) it uses Python
built-in Boolean-to-string conversion, which results in capitalized
'True'/'False' values; unfortunately, the update engine expects
a lowercase value.

BUG=chromium:243011
TEST=IsDeltaPayload field set correctly

Change-Id: Idb8c2527ed6c55af676aad868f29e3e908fc4967
Reviewed-on: https://gerrit.chromium.org/gerrit/56297
Tested-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
diff --git a/autoupdate_lib.py b/autoupdate_lib.py
index d241588..36be0ca 100644
--- a/autoupdate_lib.py
+++ b/autoupdate_lib.py
@@ -26,7 +26,7 @@
         sha256="%(sha256)s"
         needsadmin="false"
         size="%(size)s"
-        IsDelta="%(is_delta_format)s"
+        IsDeltaPayload="%(is_delta_format)s"
         status="ok"
         %(extra_attr)s/>
     </app>
@@ -53,7 +53,7 @@
               ChromeOSVersion="9999.0.0"
               sha256="%(sha256)s"
               needsadmin="false"
-              IsDelta="%(is_delta_format)s"
+              IsDeltaPayload="%(is_delta_format)s"
               %(extra_attr)s />
           </actions>
         </manifest>
@@ -143,7 +143,7 @@
   (codebase, filename) = os.path.split(url)
   response_values['codebase'] = codebase
   response_values['filename'] = filename
-  response_values['is_delta_format'] = is_delta_format
+  response_values['is_delta_format'] = str(is_delta_format).lower()
   extra_attributes = []
   if critical_update:
     # The date string looks like '20111115' (2011-11-15). As of writing,