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/Gmail.py

@@ -110,6 +110,11 @@ class GmailFolder(IMAPFolder):
         else:
             return set()
 
+    # Interface from BaseFolder
+    def msglist_item_initializer(self, uid):
+        return {'uid': uid, 'flags': set(), 'labels': set(), 'time': 0}
+
+
     # TODO: merge this code with the parent's cachemessagelist:
     # TODO: they have too much common logics.
     def cachemessagelist(self):
@@ -152,6 +157,7 @@ class GmailFolder(IMAPFolder):
                                           minor = 1)
             else:
                 uid = long(options['UID'])
+                self.messagelist[uid] = self.msglist_item_initializer(uid)
                 flags = imaputil.flagsimap2maildir(options['FLAGS'])
                 m = re.search('\(([^\)]*)\)', options['X-GM-LABELS'])
                 if m: