[AIRFLOW-2994] Fix flatten_results for BigQueryOperator (apache#3829)

Author: criccomini

airflow/contrib/hooks/bigquery_hook.py

@@ -476,7 +476,7 @@ def run_query(self,
                   destination_dataset_table=False,
                   write_disposition='WRITE_EMPTY',
                   allow_large_results=False,
-                  flatten_results=False,
+                  flatten_results=None,
                   udf_config=False,
                   use_legacy_sql=None,
                   maximum_billing_tier=None,

airflow/contrib/operators/bigquery_operator.py

@@ -106,7 +106,7 @@ def __init__(self,
                  destination_dataset_table=False,
                  write_disposition='WRITE_EMPTY',
                  allow_large_results=False,
-                 flatten_results=False,
+                 flatten_results=None,
                  bigquery_conn_id='bigquery_default',
                  delegate_to=None,
                  udf_config=False,