folder/IMAP, repository/IMAP: make encoding conditional on utf_8_support

When utf_8_support is False (the default, standard RFC 3501 mode),
folder names received from the server are in Modified UTF-7 and must
be kept in that encoding internally.  When utf_8_support is True,
names are decoded to UTF-8 for internal use.

Previous code decoded unconditionally in IMAPFolder.__init__, which
would corrupt non-ASCII names received as Modified UTF-7 when
utf_8_support is False.  The inverse problem existed in
getfullIMAPname() and the three encode_mailbox_name() call sites in
IMAPRepository: they always converted UTF-8 → Modified UTF-7 before
sending to the server, which is wrong when utf_8_support is False
(names are already in Modified UTF-7 and must not be double-encoded).

Fix by applying the same conditional pattern consistently:

  if account.utf_8_support:
      name = imaputil.utf8_IMAP(name)   # UTF-8 → Modified UTF-7
  return imaputil.foldername_to_imapname(name)

This is applied in:
  - IMAPFolder.__init__       (decode on receive)
  - IMAPFolder.getfullIMAPname (encode before SELECT)
  - IMAPRepository.getfolders  (folderincludes SELECT)
  - IMAPRepository.deletefolder
  - IMAPRepository.makefolder_single

encode_mailbox_name() (which always assumed UTF-8 input) is removed
as it is no longer used anywhere.

Based on patch by Etienne Buira <etienne.buira@free.fr>

offlineimap/folder/IMAP.py

@@ -41,7 +41,7 @@ class IMAPFolder(BaseFolder):
         #    a folder object from a locally available utf_8 name)
         # In any case the given name is first dequoted.
         name = imaputil.dequote(name)
-        if decode:
+        if decode and repository.account.utf_8_support:
             name = imaputil.IMAP_utf8(name)
         self.sep = imapserver.delim
         super(IMAPFolder, self).__init__(name, repository)
@@ -82,7 +82,10 @@ class IMAPFolder(BaseFolder):
             imapobj.select(self.getfullIMAPname(), readonly=True, force=force)
 
     def getfullIMAPname(self):
-        return imaputil.encode_mailbox_name(self.getfullname())
+        name = self.getfullname()
+        if self.repository.account.utf_8_support:
+            name = imaputil.utf8_IMAP(name)
+        return imaputil.foldername_to_imapname(name)
 
     # Interface from BaseFolder
     def suggeststhreads(self):

offlineimap/imaputil.py

@@ -449,10 +449,6 @@ def utf7m_search_function(name):
 codecs.register(utf7m_search_function)
 
 
-def encode_mailbox_name(mbox_name):
-    return foldername_to_imapname(utf8_IMAP(mbox_name))
-
-
 def foldername_to_imapname(folder_name):
     """
     This function returns the folder_name ready to send to the

offlineimap/repository/IMAP.py

@@ -752,7 +752,10 @@ class IMAPRepository(BaseRepository):
             try:
                 for foldername in self.folderincludes:
                     try:
-                        imapobj.select(imaputil.encode_mailbox_name(foldername),
+                        imap_name = foldername
+                        if self.account.utf_8_support:
+                            imap_name = imaputil.utf8_IMAP(foldername)
+                        imapobj.select(imaputil.foldername_to_imapname(imap_name),
                                        readonly=True)
                     except OfflineImapError as exc:
                         # couldn't select this folderinclude, so ignore folder.
@@ -816,7 +819,9 @@ class IMAPRepository(BaseRepository):
     def deletefolder(self, foldername):
         """Delete a folder on the IMAP server."""
 
-        foldername = imaputil.encode_mailbox_name(foldername)
+        if self.account.utf_8_support:
+            foldername = imaputil.utf8_IMAP(foldername)
+        foldername = imaputil.foldername_to_imapname(foldername)
         imapobj = self.imapserver.acquireconnection()
         try:
             result = imapobj.delete(foldername)
@@ -877,7 +882,9 @@ class IMAPRepository(BaseRepository):
             return
         imapobj = self.imapserver.acquireconnection()
         try:
-            foldername = imaputil.encode_mailbox_name(foldername)
+            if self.account.utf_8_support:
+                foldername = imaputil.utf8_IMAP(foldername)
+            foldername = imaputil.foldername_to_imapname(foldername)
 
             result = imapobj.create(foldername)
             if result[0] != 'OK':