utf8foldernames: support --delete-folder with UTF-8 folder name

Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/505
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/repository/IMAP.py

@@ -524,6 +524,8 @@ class IMAPRepository(BaseRepository):
     def deletefolder(self, foldername):
         """Delete a folder on the IMAP server."""
 
+        if self.account.utf_8_support:
+            foldername = imaputil.utf8_IMAP(foldername)
         imapobj = self.imapserver.acquireconnection()
         try:
             result = imapobj.delete(foldername)