Fix create_backup and get_backup.

[BoostCookie]

Fixes https://github.com/finish06/pyunifi/issues/61 and more.

For create_backup the suggested solution from https://github.com/finish06/pyunifi/issues/61 was taken.

get_backup still failed, because if response != 200: was used instead of if response.status_code != 200:.

The README.md was updated to contain both arguments for get_backup.

[numo68]

any chance getting this merged? I confirm that it fixes the problem.