def merge_rules(default_rules, custom_rules):
    """
    Merge both rule set. Rules defined in 'custom_rules', also present in 'default_rules', will be overridden.
    If a new rule is present in custom_rules, not present in default_rules, it will be added.
    :param default_rules: base file object of rules.
    :type default_rules: dict
    :param custom_rules: override file object of rules.
    :type custom_rules: dict
    :return: final rule
    :rtype: dict
    """

    for rule in custom_rules['Headers']:
        default_rules['Headers'][rule] = custom_rules['Headers'][rule]

    return default_rules