Fixed a realm wobble with DIGEST-MD5 authentication.

Author: Ben Weaver

sasl/auth.py

@@ -88,10 +88,12 @@ def __init__(self, pass_type, users, user, passwd, serv_type, host, authzid=None
         self._user = user
         self._passwd = passwd
         self._authzid = authzid
-        self._realm = realm
         self._serv_type = serv_type
         self._host = host
 
+        if realm:
+            self.realm = realm
+
         ## Install
         self._entities = dict(
             (k, pass_type.make(self, k, p))