Properly manipulate contents of messagelist for folder

Create initializer function that puts default values to all fields
of message list item.  Fix all code that directly assigns some hash
to the elements of messagelist: for direct assignments only initializer
is now permitted, all other modification are done in-place.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/folder/GmailMaildir.py

@@ -56,6 +56,12 @@ class GmailMaildirFolder(MaildirFolder):
                 return True
         return False  #Nope, nothing changed
 
+
+    # Interface from BaseFolder
+    def msglist_item_initializer(self, uid):
+        return {'flags': set(), 'labels': set(), 'filename': '/no-dir/no-such-file/', 'mtime': 0}
+
+
     def cachemessagelist(self):
         if self.messagelist is None:
             self.messagelist = self._scanfolder()