plugin XEP-0045: fixed bad default parameter

src/plugins/plugin_xep_0045.py

@@ -380,10 +380,12 @@ class XEP_0045(object):
         # > /usr/local/lib/python2.7/dist-packages/twisted/internet/defer.py(480)_startRunCallbacks()
         # -> raise AlreadyCalledError(extra)
 
-    def _join(self, room_jid_s, nick, options={}, profile_key=C.PROF_KEY_NONE):
+    def _join(self, room_jid_s, nick, options=None, profile_key=C.PROF_KEY_NONE):
         """join method used by bridge: use the join method, but doesn't return any deferred
         @return: unicode (the room bare)
         """
+        if options is None:
+            options = {}
         profile = self.host.memory.getProfileName(profile_key)
         if not self.checkClient(profile):
             return