As an application developer I only want to load the updated data from Wikibase instance, so I can update it in my app only when data changes
See `If-None-Match` and `If-Modified-Since` parameters in the specification: https://gerrit.wikimedia.org/r/773447
**BDD**
As an API user
GIVEN there is a newer version of item data than revision ID N
WHEN I make a GET request to /entities/items/{item_id}
AND I provide N as the revision ID in If-None-Match header
THEN I receive 200 HTTP response from the API
AND response contains item data
As an API user
GIVEN The most recent version of an item is N
WHEN I make a GET request to /entities/items/{item_id}
AND I provide N as the revision ID in If-None-Match header
THEN I receive 304 HTTP response from the API
AND response headers contain the item metadata as indicated in the specs
As an API user
GIVEN there is a newer version of item data than published on or before the timestamp Y
WHEN I make a GET request to /entities/items/{item_id}
AND I provide Y as a timestamp in If-Modified-Since header
THEN I receive 200 HTTP response from the API
AND response contains item data
As an API user
GIVEN The most recent version of an item is the version published on or before the timestamp Y
WHEN I make a GET request to /entities/items/{item_id}
AND I provide Y as a timestamp in If-Modified-Since header
THEN I receive 304 HTTP response from the API
AND response headers contain the item metadata as indicated in the specs
**Note:**
- weak comparison of ETags is out of scope of this story, and will be handled separately.
- [weak comparison](https://developer.mozilla.org/en-US/docs/Web/HTTP/Conditional_requests#weak_validation) of ETags is to consider two revisions with the same item data (ID, labels, descriptions, aliases, statements and sitelinks) identical, i.e. ignoring the date of the revision in comparison
- in weak comparison, statements are considers equal if their whole structure is the same (i.e. including GUIDs)
- in other words, in weak comparision items are considered equal if the "diff" between their json structures is the same