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/LocalStatus.py

@@ -190,13 +190,16 @@ class LocalStatusFolder(BaseFolder):
                 os.close(fd)
 
     # Interface from BaseFolder
-    def savemessage(self, uid, content, flags, rtime, mtime=0, labels=set()):
+    def savemessage(self, uid, content, flags, rtime, mtime=0, labels=None):
         """Writes a new message, with the specified uid.
 
         See folder/Base for detail. Note that savemessage() does not
         check against dryrun settings, so you need to ensure that
         savemessage is never called in a dryrun mode."""
 
+        if labels is None:
+            labels = set()
+
         if uid < 0:
             # We cannot assign a uid.
             return uid