update origin dict with override dict recursively
    e.g. origin_dict = {'a': 1, 'b': {'c': 2, 'd': 4}}
         override_dict = {'b': {'c': 3}}
    return: {'a': 1, 'b': {'c': 3, 'd': 4}}

def deep_update_dict(origin_dict, override_dict):
    """ update origin dict with override dict recursively
    e.g. origin_dict = {'a': 1, 'b': {'c': 2, 'd': 4}}
         override_dict = {'b': {'c': 3}}
    return: {'a': 1, 'b': {'c': 3, 'd': 4}}
    """
    if not override_dict:
        return origin_dict

    for key, val in override_dict.items():
        if isinstance(val, dict):
            tmp = deep_update_dict(origin_dict.get(key, {}), val)
            origin_dict[key] = tmp
        elif val is None:
            # fix #64: when headers in test is None, it should inherit from config
            continue
        else:
            origin_dict[key] = override_dict[key]

    return origin_dict