**Apache Airflow version**:1.10.12
**Environment**: google cloud composer

**What happened**:
When using GoogleCloudStorageToBigQueryOperator and providing string as parameter source_objects, the process is iterating on a the string as a valid list.
For example - 
`cloud_storage_to_bigquery = GoogleCloudStorageToBigQueryOperator(
        bucket = 'bucket',
        source_objects = 'abc',
    )`
Will result in looking into the sources: bucket/a, bucket/b, bucket/c.

**What you expected to happen**:
Throw an error on type (string instead of list). 

tests/providers/google/cloud/transfers/test_gcs_to_bigquery.py
@mock.patch('airflow.providers.google.cloud.transfers.gcs_to_bigquery.BigQueryHook')
    def test_execute_explicit_project_legacy(self, bq_hook):
        operator = GCSToBigQueryOperator(
            task_id=TASK_ID,
            bucket=TEST_BUCKET,
            source_objects=TEST_SOURCE_OBJECTS,
            destination_project_dataset_table=TEST_EXPLICIT_DEST,
            max_id_key=MAX_ID_KEY,
        )
        
        bq_hook.return_value.get_conn.return_value.cursor.return_value.use_legacy_sql = True
        operator.execute(None)
        bq_hook.return_value.get_conn.return_value.cursor.return_value.execute.assert_called_once_with(
            "SELECT MAX(id) FROM [test-project.dataset.table]"
        )
    @mock.patch('airflow.providers.google.cloud.transfers.gcs_to_bigquery.BigQueryHook')
    def test_execute_explicit_project(self, bq_hook):