Fix imap date on POP3

[bytestream]

$tmp['Date'] is an instance of Horde_Mime_Headers_Date which results in an incorrect type passed to setImapDate():

/**
 * Set IMAP internal date.
 *
 * @param mixed $date  Either a Horde_Imap_Client_DateTime object or a date string.
 */
public function setImapDate($date)
{
    $this->_data[Horde_Imap_Client::FETCH_IMAPDATE] = is_object($date)
        ? $date
        : new Horde_Imap_Client_DateTime($date);
}

Call getImapDate() on a POP3 socket using a fetch query of the below to replicate:

$query = new Horde_Imap_Client_Fetch_Query;
$query->imapDate();