swarming: add transaction_id to tasks.new request

appengine/swarming/server/task_request.py

Index: appengine/swarming/server/task_request.py
diff --git a/appengine/swarming/server/task_request.py b/appengine/swarming/server/task_request.py
index 8e2ecd98f8b398ea310a6ab108f92c6522d0cf48..dfdd39cafcfdec68b944d506d4c7e384e9997a99 100644
--- a/appengine/swarming/server/task_request.py
+++ b/appengine/swarming/server/task_request.py
@@ -659,11 +659,12 @@ class TaskRequest(ndb.Model):
         acl.is_privileged_user() or
         self.authenticated == auth.get_current_identity())
 
-  def to_dict(self):
+  def to_dict(self, exclude=None):
     """Converts properties_hash to hex so it is json serializable."""
     # to_dict() doesn't recurse correctly into ndb.LocalStructuredProperty!
-    out = super(TaskRequest, self).to_dict(
-        exclude=['pubsub_auth_token', 'properties', 'service_account_token'])
+    exclude = (exclude or [])[:]
+    exclude.extend(['pubsub_auth_token', 'properties', 'service_account_token'])
+    out = super(TaskRequest, self).to_dict(exclude=exclude)
     out['properties'] = self.properties.to_dict()
     properties_hash = out['properties_hash']
     out['properties_hash'] = (