Default 'limit' to 25 in v3
Fix error getting items in empty collection
Accept full JSON as an alternative to editable JSON

As part of this, the API now takes a JSON array directly for
multi-object writes instead of an object with a 'collections' /
'searches' / 'items' array. This will break existing multi-object
writes, but we get this in return:

https://www.zotero.org/support/dev/web_api/v3/write_requests#rest_approach