**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). 

airflow/providers/google/cloud/transfers/gcs_to_bigquery.py
if self.external_table:
            cursor.create_external_table(
                external_project_dataset_table=self.destination_project_dataset_table,
                schema_fields=schema_fields,
                source_uris=source_uris,
                source_format=self.source_format,
                compression=self.compression,
                skip_leading_rows=self.skip_leading_rows,
                field_delimiter=self.field_delimiter,
                max_bad_records=self.max_bad_records,
                quote_character=self.quote_character,
                ignore_unknown_values=self.ignore_unknown_values,
                allow_quoted_newlines=self.allow_quoted_newlines,
                allow_jagged_rows=self.allow_jagged_rows,
                encoding=self.encoding,
                src_fmt_configs=self.src_fmt_configs,
                encryption_configuration=self.encryption_configuration,
                labels=self.labels,
                description=self.description,
            )
        else:
            cursor.run_load(
                destination_project_dataset_table=self.destination_project_dataset_table,
                schema_fields=schema_fields,
                source_uris=source_uris,
                source_format=self.source_format,
                autodetect=self.autodetect,
                create_disposition=self.create_disposition,
                skip_leading_rows=self.skip_leading_rows,
                write_disposition=self.write_disposition,