Update transaction.py

Author: ddrondo

django_mongodb_backend/transaction.py

@@ -21,6 +21,7 @@ def __init__(self, using):
 
     def __enter__(self):
         connection = get_connection(self.using)
+        connection.ensure_connection()
         if connection.in_atomic_block_mongo:
             # Track the number of nested atomic() calls.
             connection.nested_atomics += 1