Removed mutable arguments

This patch remves the set() mutable argument as default value,
sets the default value to None and check if the argument is none
in the code to call set().

offlineimap/folder/Maildir.py

@@ -270,7 +270,7 @@ class MaildirFolder(BaseFolder):
         filepath = os.path.join(self.getfullname(), filename)
         return os.path.getmtime(filepath)
 
-    def new_message_filename(self, uid, flags=set(), date=None):
+    def new_message_filename(self, uid, flags=None, date=None):
         """Creates a new unique Maildir filename
 
         :param uid: The UID`None`, or a set of maildir flags
@@ -278,6 +278,9 @@ class MaildirFolder(BaseFolder):
         :param flags: (optional) Date
         :returns: String containing unique message filename"""
 
+        if flags is None:
+            flags = set()
+
         timeval, timeseq = _gettimeseq(date)
         uniq_name = '%d_%d.%d.%s,U=%d,FMD5=%s%s2,%s' % \
                     (timeval, timeseq, os.getpid(), socket.gethostname(),