Support jabberd2 SASL with really empty response

Despite http://xmpp.org/rfcs/rfc3920.html#rfc.section.6.2, jabberd version 2.2.14 cannot accept the typical "<response xmlns="urn:ietf:params:xml:ns:xmpp-sasl">=</response>".  Instead it must be truly empty, so we force an empty response for this stanza only.

sleekxmpp/features/feature_mechanisms/mechanisms.py

@@ -215,6 +215,8 @@ class FeatureMechanisms(BasePlugin):
             self.attempted_mechs.add(self.mech.name)
             self.xmpp.disconnect()
         else:
+            if resp.get_value() == '':
+                resp.del_value()
             resp.send(now=True)
 
     def _handle_success(self, stanza):