APIv3 change: dateModified handling

[Dan Stillman]

We've made a small change to how the Date Modified field is handled in
the API.

See
https://github.com/zotero/dataserver/commit/4f4bf3f20223f30066d5075abfd88d8283072119
for the relevant details. Basically, if you rely on the server updating
the Date Modified timestamp automatically on an item write, you'll need
to unset the 'dateModified' property in the uploaded JSON. Otherwise,
the provided timestamp will be used, even if it matches the item's
existing Date Modified value. Previously, if a timestamp was provided
that matched the existing one, the field would be updated.

This change is currently in effect only for user agents that include
"Zotero" or "Firefox" (because it helps us avoid unwanted timestamp
updates in Zotero 5.0), but we'll be applying it to all v3 clients in a
few weeks, so update your code by June 8, 2016 if you want the old behavior.

API versions 1 and 2 are unaffected.