API: /my_files/ does not overwrite existing file

[moi90]

If I upload the a file with the same name and tag again, the server uses the old copy, if available:

UserDir.py#L48

I guess the check (if dest_path.exists()) is there to save time. But it is too late to save time at this point, anyways: The data was already transferred completely, as the multipart-encoded request body was already deserialized at this point. And it is wrong: When the file upload request finishes without error, I expect the sent data to be written.

The comment says "Should be an option", but I think this is never needed. If someone wants to skip existing files, they can GET /my_files/. (See #56.)