Clear IMAP capabilities on connection close

Fixes the state on connection close, so when you reconnect there's use
of old capabiltieis. This fixes the following scenario:
- user connects to imap and authenticates using LOGIN command,
  after this capabilities may contain LOGINDISABLED
- user disconnects
- in the same request user connects again and authenticates as before
  but it can't because of the wrong LOGINDISABLED state.

program/lib/Roundcube/rcube_imap_generic.php

@@ -1165,6 +1165,7 @@ public function closeConnection()
         }
 
         $this->closeSocket();
+        $this->clearCapability();
     }
 
     /**