**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
destination_project_dataset_table=mock.ANY,
            schema_fields=mock.ANY,
            source_uris=mock.ANY,
            source_format=mock.ANY,
            autodetect=mock.ANY,
            create_disposition=mock.ANY,
            skip_leading_rows=mock.ANY,
            write_disposition=mock.ANY,
            field_delimiter=mock.ANY,
            max_bad_records=mock.ANY,
            quote_character=mock.ANY,
            ignore_unknown_values=mock.ANY,
            allow_quoted_newlines=mock.ANY,
            allow_jagged_rows=mock.ANY,
            encoding=mock.ANY,
            schema_update_options=mock.ANY,
            src_fmt_configs=mock.ANY,
            time_partitioning=mock.ANY,
            cluster_fields=mock.ANY,
            encryption_configuration=mock.ANY,
            labels=mock.ANY,
            description=DESCRIPTION,
        )
    @mock.patch('airflow.providers.google.cloud.transfers.gcs_to_bigquery.BigQueryHook')
    def test_labels_external_table(self, bq_hook):
        operator = GCSToBigQueryOperator(
            task_id=TASK_ID,
            bucket=TEST_BUCKET,
            source_objects=TEST_SOURCE_OBJECTS,
            destination_project_dataset_table=TEST_EXPLICIT_DEST,