Fix consistency checks in ConfdFilterCallback

Commit 49b3fdac added consistency checks, but these are wrongly triggered
for old responses - we need to make sure to check that we have the same
serial.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

lib/confd/client.py

@@ -424,7 +424,8 @@ class ConfdFilterCallback:
       # else: different content, pass up a second answer
     else:
       # older or same-version answer (duplicate or outdated, filter)
-      if up.server_reply.answer != self._answers[salt].answer:
+      if (up.server_reply.serial == self._answers[salt].serial and
+          up.server_reply.answer != self._answers[salt].answer):
         self.consistent[salt] = False
       filter_upcall = True
       self._LogFilter(salt, up.server_reply, self._answers[salt])