Build the Expected parameters from a dict

def build_expected(dynamizer, expected):
    """ Build the Expected parameters from a dict """
    ret = {}
    for k, v in six.iteritems(expected):
        if is_null(v):
            ret[k] = {
                'Exists': False,
            }
        else:
            ret[k] = {
                'Exists': True,
                'Value': dynamizer.encode(v),
            }
    return ret