Fix for XEP-0030 'Result-set for all items'

* As per http://xmpp.org/extensions/xep-0030.html#items Example 12, a
  disco#items request "... MUST return its list of publicly-available items,
  or return an error."
* This will have an extra 'node' attribute when requesting all items, but I
  don't think this will matter.
* After this patch, Pidgin will successfully see ad-hoc commands (as of 2.10.9)