Fix tag registering for infinite ttl.

Tag that had infinite TTL and with a periodic refresh didn't had their
registration done in case off None -> not None value and vice versa.

ccnode/tags.py

@@ -65,13 +65,10 @@ class Tag(object):
         """Called when the tag value may change."""
         prev_value = self.value
         self.calculate_value()
-        # if tag is not pushed
-        # if self.ttl is not None:
-        if self.ttl != -1:
-            return
+
         if prev_value == self.value:
             return
-        if prev_value is None:
+        elif prev_value is None:
             # we need to register tag again
             if self.sub_id == '__main__':
                 logger.debug('Register tag %s', self.name)
@@ -88,6 +85,10 @@ class Tag(object):
             else:
                 logger.debug('Unregister sub tag %s.%s', self.sub_id, self.name)
                 self.db.rpc_unregister_sub_tag(self.sub_id, self.name)
+        # if tag is not pushed
+        # if self.ttl is not None:
+        elif self.ttl != -1:
+            return
         else:
             # update the tag value
             logger.debug('Update tag value %s', self.name)