def _normalize_headers(_headers):
    """This function normalizes the HTTP headers to ensure that the keys and values are all lowercase.

    .. versionchanged:: 2.7.5
       The function was updated to ensure that authentication/authorization tokens would not be altered.

    .. versionadded:: 2.7.4

    :param _headers: The headers dictionary
    :type  _headers: dict
    :returns: The normalized headers dictionary
    """
    _normalized_headers = {}
    _auth_keys = ['li-api-session-key', 'authorization']
    for _header_key, _header_value in _headers.items():
        if _header_key.lower() in _auth_keys:
            _normalized_headers[_header_key] = _header_value
        elif isinstance(_header_value, str):
            _normalized_headers[_header_key.lower()] = _header_value.lower()
        elif any((isinstance(_header_value, list), isinstance(_header_value, tuple))):
            _new_iterable = []
            for _inner_value in _header_value:
                _inner_value = _inner_value.lower() if isinstance(_inner_value, str) else _inner_value
                _new_iterable.append(_inner_value)
            _new_iterable = tuple(_new_iterable) if isinstance(_header_value, tuple) else _new_iterable
            _normalized_headers[_header_key.lower()] = _new_iterable
        else:
            _header_key = _header_key.lower() if isinstance(_header_key, str) else _header_key
            _normalized_headers[_header_key] = _header_value
    return _normalized_headers