Update the "delete a deleted object" test for new API behaviour

At one point the PopIt API returned a 404 if you tried to DELETE
a non-existent object, but in the current version it'll return
204 No Content.  Update the test that checks for this case.
Stop slumber from appending slashes to URLs

This causes object deletion, for example, to fail since the URL
ends in an ID - PopIt interprets a trailing slash as part of the
ID.
The "comment" field of a link is now called "note" in the Popolo spec