BUG: Support for server capabilities

Server capabilities are returned as list of bytes. We need convert them
to list of strings.

This patch do it. Probably you must recreate your cache after this patch.

offlineimap/imapserver.py

@@ -595,7 +595,9 @@ class IMAPServer:
             # update capabilities after login, e.g. gmail serves different ones
             typ, dat = imapobj.capability()
             if dat != [None]:
-                imapobj.capabilities = tuple(dat[-1].upper().split())
+                # Get the capabilities and convert them to string from bytes
+                s_dat = [x.decode('utf-8') for x in dat[-1].upper().split()]
+                imapobj.capabilities = tuple(s_dat)
 
             if self.delim is None:
                 listres = imapobj.list(self.reference, '""')[1]