Save is used to either create new entries or to save changes to existing
ones. If we have a link rel="edit", we are changing an existing entry. In
this case we re-serialize the entry and PUT it to the edit URI, checking
for a 200 OK result.
For posting new entries, you must specify the $postUri parameter to
save() to tell the object where to post itself. We use $postUri and POST
the serialized entry there, checking for a 201 Created response. If the
insert is successful, we then parse the response from the POST to get any
values that the server has generated: an id, an updated time, and its new
link rel="edit".