Commit 6b28071b0f makes the check once nametrans is applied. This is wrong
because separators are valid characters in names returned by nametrans. E.g:
"Sent" -> "[Gmail]/Sent"
Make the check against the raw name of the folder, instead.
Github-fix: https://github.com/OfflineIMAP/offlineimap/issues/353
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>