Allow disco info/items handlers to return full Iq stanzas.

Only allowing handlers to return a DiscoInfo/DiscoItem stanza works
for the majority of cases, but does not allow for the addition of
an RSM stanza, or other extensions.

An Iq stanza returned by a handler must already be configured as
a reply.

sleekxmpp/plugins/xep_0030/disco.py

@@ -551,6 +551,9 @@ class xep_0030(base_plugin):
                                           jid,
                                           iq['disco_info']['node'],
                                           iq)
+            if isinstance(info, Iq):
+                info.send()
+            else:
                 iq.reply()
                 if info:
                     info = self._fix_default_info(info)
@@ -581,6 +584,9 @@ class xep_0030(base_plugin):
                                           jid,
                                           iq['disco_items']['node'],
                                           iq)
+            if isinstance(items, Iq):
+                items.send()
+            else:
                 iq.reply()
                 if items:
                     iq.set_payload(items.xml)