When PATCHing an item, Date Modified is not updated

[tnajdek]

This could be fixed by either switching to POSTing for item updates but that's slower and wasteful. Alternatively, maybe, we could be reading Date header from the response - this would only work if we can be sure that field value has indeed changed from the one server-side, which we already do because of https://github.com/zotero/dataserver/issues/81).

We would need an exception for tags until https://github.com/zotero/dataserver/issues/165 is resolved.