Fix line-endings in tests on Windows
EDIT:
~~Fix test_delete_from_hub on Windows by passing explicit encoding.~~
Fix test_delete_from_hub and test_xgetsize_private by uploading the README file content directly (encoding the string), instead of writing a local file and uploading it.

Note that local files created on Windows will have "\r\n" line endings, instead of "\n".
These are no longer transformed to "\n" by the Hub.

Fix #6856.