fixes for new itemset interface

This commit addresses the following issues, which broke the itemset
interface:

    1. The server was updated to accept the contents: pointer, but the
       client was not. The client and its specs have been updated.

    2. The server used to assume content-type: application/json, but
       no longer did after b53b8da8. The server has been patched to
       assume application/json if nothing is provided, and the client
       has been updated to provide it by default.

Also included is some light refactoring for the itemset class,
including the ability to request multiple organizations, topics, and
ids with the same itemset object.