Fix stanza clobbering when replying to errors.

If a stanza handler raised an exception, the exception was processed
and replied by the modified stanza, not a stanza with the original
content.

A copy is now made before handler processing, and if an exception occurs
it is the copy that processes the exception using the original content.

sleekxmpp/stanza/message.py

@@ -97,7 +97,7 @@ class Message(RootStanza):
             clear -- Indicates if existing content should be removed
                      before replying. Defaults to True.
         """
-        StanzaBase.reply(self)
+        StanzaBase.reply(self, clear)
         if self['type'] == 'groupchat':
             self['to'] = self['to'].bare