item_upsert never updates

I can't find a way to call `item_upsert` such that it updates rather than creates a new item.

After logging in:
```r
test_item_1 <- item_upsert(title="books1")
test_item_2 <- item_upsert(title="books1")
test_item_1$id == test_item_2$id
## [1] FALSE
test_item_3 <- item_upsert(title="books2")
test_item_4 <- item_upsert(title="books2", info=list(contacts = list(list(name = "Suzy"))))
test_item_3$id == test_item_4$id
## [1] FALSE
```

I had expected the tests to yield `TRUE` - i.e., `item_upsert` should update rather than create an existing item if the titles match. The two calls to `item_upsert` in the Examples section of the help document also produce two distinct items, despite the claim that the second call "updates this time".