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/IMAP.py

@@ -202,6 +202,10 @@ class IMAPFolder(BaseFolder):
 
         return msgsToFetch
 
+    # Interface from BaseFolder
+    def msglist_item_initializer(self, uid):
+        return {'uid': uid, 'flags': set(), 'time': 0}
+
 
     # Interface from BaseFolder
     def cachemessagelist(self):
@@ -239,6 +243,7 @@ class IMAPFolder(BaseFolder):
                                           minor = 1)
             else:
                 uid = long(options['UID'])
+                self.messagelist[uid] = self.msglist_item_initializer(uid)
                 flags = imaputil.flagsimap2maildir(options['FLAGS'])
                 rtime = imaplibutil.Internaldate2epoch(messagestr)
                 self.messagelist[uid] = {'uid': uid, 'flags': flags, 'time': rtime}
@@ -641,7 +646,8 @@ class IMAPFolder(BaseFolder):
             if imapobj: self.imapserver.releaseconnection(imapobj)
 
         if uid: # avoid UID FETCH 0 crash happening later on
-            self.messagelist[uid] = {'uid': uid, 'flags': flags}
+            self.messagelist[uid] = self.msglist_item_initializer(uid)
+            self.messagelist[uid]['flags'] = flags
 
         self.ui.debug('imap', 'savemessage: returning new UID %d' % uid)
         return uid