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/Base.py

@@ -524,13 +524,16 @@ class BaseFolder:
 
         raise NotImplementedError
 
-    def savemessagelabels(self, uid, labels, ignorelabels=set(), mtime=0):
+    def savemessagelabels(self, uid, labels, ignorelabels=None, mtime=0):
         """Sets the specified message's labels to the given set.
 
         Note that this function does not check against dryrun settings,
         so you need to ensure that it is never called in a
         dryrun mode."""
 
+        if ignorelabels is None:
+            ignorelabels = set()
+
         raise NotImplementedError
 
     def addmessagelabels(self, uid, labels):