Fix md5 folder generation wanting unicode

Using standard offlineimap without specific utf8 nametrans makes
offlineimap crash when generating md5 of the folder because the
foldername is already an str.

Signed-off-by: Stéphane Albert <sheeprine@oh.its.fake.nullplace.com>

offlineimap/folder/Maildir.py

@@ -71,7 +71,7 @@ class MaildirFolder(BaseFolder):
         # Everything up to the first comma or colon (or ! if Windows):
         self.re_prefixmatch = re.compile('([^'+ self.infosep + ',]*)')
         # folder's md, so we can match with recorded file md5 for validity.
-        self._foldermd5 = md5(self.getvisiblename().encode('utf-8')).hexdigest()
+        self._foldermd5 = md5(self.getvisiblename()).hexdigest()
         # Cache the full folder path, as we use getfullname() very often.
         self._fullname = os.path.join(self.getroot(), self.getname())
         # Modification time from 'Date' header.