Set a keys expire at given UNIX time

    CLI Example:

    .. code-block:: bash

        salt '*' redis.expireat foo 1400000000

def expireat(key, timestamp, host=None, port=None, db=None, password=None):
    '''
    Set a keys expire at given UNIX time

    CLI Example:

    .. code-block:: bash

        salt '*' redis.expireat foo 1400000000
    '''
    server = _connect(host, port, db, password)
    return server.expireat(key, timestamp)