[AIRFLOW-618] Cast DateTimes to avoid sqllite errors

For some reason, sqllite does not always match
datetimes correctly.
This causes unit test failures, in particular the
trigger_dagrun test.
Casting the datetime explicitly appears to solve
the issue.

Closes apache#1871 from jlowin/failing-sqlite-test

airflow/models.py

@@ -3650,9 +3650,11 @@ def refresh_from_db(self, session=None):
         """
         DR = DagRun
 
+        exec_date = func.cast(self.execution_date, DateTime)
+
         dr = session.query(DR).filter(
             DR.dag_id == self.dag_id,
-            DR.execution_date == self.execution_date,
+            func.cast(DR.execution_date, DateTime) == exec_date,
             DR.run_id == self.run_id
         ).one()