**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
bucket=TEST_BUCKET,
            source_objects=TEST_SOURCE_OBJECTS,
            destination_project_dataset_table=TEST_EXPLICIT_DEST,
            description=DESCRIPTION,
            external_table=True,
        )
        operator.execute(None)
        
        bq_hook.return_value.get_conn.return_value.cursor.return_value.create_external_table. \
            assert_called_once_with(
                external_project_dataset_table=mock.ANY,
                schema_fields=mock.ANY,
                source_uris=mock.ANY,
                source_format=mock.ANY,
                compression=mock.ANY,
                skip_leading_rows=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,
                src_fmt_configs=mock.ANY,
                encryption_configuration=mock.ANY,
                labels=mock.ANY,
                description=DESCRIPTION,
            )