IMAP cachefolder: Fix returning None on select

We rely on the number of mails being returned by the imapobj.select()
call, however that only happens if we "force" a real select() to occur.
Pass in the force parameter that I dropped earlier (we did not make use
of the return value when I dropped it, that is how it slipped through).

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/IMAP.py

@@ -120,7 +120,7 @@ class IMAPFolder(BaseFolder):
 
         imapobj = self.imapserver.acquireconnection()
         try:
-            res_type, imapdata = imapobj.select(self.getfullname(), True)
+            res_type, imapdata = imapobj.select(self.getfullname(), True, True)
             if imapdata == [None] or imapdata[0] == '0':
                 # Empty folder, no need to populate message list
                 return