Error when trying fetch messages from container

[kekdev]

imap_getmailboxes returns also mailbox container. It needs additional attributes check for LATT_NOSELECT according to documentation

Gmail as example
PHP Fatal error: Uncaught ErrorException: Unknown: [NONEXISTENT] Unknown Mailbox: [Gmail] (now in authenticated state) (Failure) (errflg=2) in Unknown:0

[Slamdunk]

With the feature implemented in #184 you can now get mailbox attributes by calling $mailbox->getAttributes(), so you can skip continer mailboxes with a code like:

if ($mailbox->getAttributes() & \LATT_NOSELECT) {
    // skip this mailbox
}

Thank you for the feedback.