Deferred DAG assignment breaks with default_args

[dimberman]

Apache Airflow version: None

Ticket was created 05/Jan/17 14:48

Kubernetes version (if you are using kubernetes) (use kubectl version):

Environment:

• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:
  What happened:

Deferred DAG assignment raises AirflowException if the dag has default_args instead of start_date:

default_args = {'start_date': datetime(2016, 1, 1)}
dag = DAG('my_dag2', default_args=default_args)
deferred_op = DummyOperator(task_id='dummy')
deferred_op.dag = dag
---------------------------------------------------------------------------
AirflowException                          Traceback (most recent call last)
<ipython-input-7-d9ff2f9c7283> in <module>()
----> 1 deferred_op.dag = dag
...
AirflowException: Task is missing the start_date parameter

What you expected to happen:

How to reproduce it:

Anything else we need to know:

Moved here from https://issues.apache.org/jira/browse/AIRFLOW-733