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/Maildir.py

@@ -191,7 +191,9 @@ class MaildirFolder(BaseFolder):
                 else:
                     uid = long(uidmatch.group(1))
             # 'filename' is 'dirannex/filename', e.g. cur/123,U=1,FMD5=1:2,S
-            retval[uid] = {'flags': flags, 'filename': filepath}
+            retval[uid] = self.msglist_item_initializer(uid)
+            retval[uid]['flags'] = flags
+            retval[uid]['filename'] = filepath
         return retval
 
     # Interface from BaseFolder
@@ -208,6 +210,12 @@ class MaildirFolder(BaseFolder):
                 return True
         return False  #Nope, nothing changed
 
+
+    # Interface from BaseFolder
+    def msglist_item_initializer(self, uid):
+        return {'flags': set(), 'filename': '/no-dir/no-such-file/'}
+
+
     # Interface from BaseFolder
     def cachemessagelist(self):
         if self.messagelist is None:
@@ -319,7 +327,9 @@ class MaildirFolder(BaseFolder):
         if rtime != None:
             os.utime(os.path.join(self.getfullname(), tmpname), (rtime, rtime))
 
-        self.messagelist[uid] = {'flags': flags, 'filename': tmpname}
+        self.messagelist[uid] = self.msglist_item_initializer(uid)
+        self.messagelist[uid]['flags'] = flags
+        self.messagelist[uid]['filename'] = tmpname
         # savemessageflags moves msg to 'cur' or 'new' as appropriate
         self.savemessageflags(uid, flags)
         self.ui.debug('maildir', 'savemessage: returning uid %d' % uid)
@@ -339,6 +349,9 @@ class MaildirFolder(BaseFolder):
         Note that this function does not check against dryrun settings,
         so you need to ensure that it is never called in a
         dryrun mode."""
+
+        assert uid in self.messagelist
+
         oldfilename = self.messagelist[uid]['filename']
         dir_prefix, filename = os.path.split(oldfilename)
         # If a message has been seen, it goes into 'cur'