Detect x-paginated in ArkindexClient.paginate

See requests#351

Requires backend#811 (closed)

Some list endpoints can be confusing as they do not always use the pagination, and paginate breaks with some ugly errors when they do not, for example with cli.paginate('ListCorpus') or cli.paginate('ListElementMetaData'). paginate could detect a x-paginated: false and properly react, faking a pagination when there is none.