Fix the uses of stanza.reply()

This is relying on the stanzas being copied for each handler. We no
longer do that for performance reasons, so instead of editing the copy
in-place, stanza.reply() now returns a new stanza.

slixmpp/stanza/presence.py

@@ -174,8 +174,9 @@ class Presence(RootStanza):
             clear -- Indicates if the stanza contents should be removed
                      before replying. Defaults to True.
         """
+        new_presence = StanzaBase.reply(self, clear)
         if self['type'] == 'unsubscribe':
-            self['type'] = 'unsubscribed'
+            new_presence['type'] = 'unsubscribed'
         elif self['type'] == 'subscribe':
-            self['type'] = 'subscribed'
-        return StanzaBase.reply(self, clear)
+            new_presence['type'] = 'subscribed'
+        return new_presence